MongooseError - Operation `users.findOne()` buffering timed out after 10000ms

My code was working before initially but I don't know why it just stopped working and gave me this error:

MongooseError: Operation `users.findOne()` buffering timed out after 10000ms
    at Timeout.<anonymous> (/Users/nishant/Desktop/Yourfolio/backend/node_modules/mongoose/lib/drivers/node-mongodb-native/collection.js:184:20)
    at listOnTimeout (internal/timers.js:549:17)
    at processTimers (internal/timers.js:492:7)

I am trying to authenticate the user by login with JWT. My client runs fine but in my backend I get this error. My backend code:

import neuron from '@yummyweb/neuronjs'
import bodyParser from 'body-parser'
import cors from 'cors'
import mongoose from 'mongoose'
import emailValidator from 'email-validator'
import passwordValidator from 'password-validator'
import User from './models/User.js'
import Portfolio from './models/Portfolio.js'
import bcrypt from 'bcryptjs'
import jwt from 'jsonwebtoken'
import auth from './utils/auth.js'

// Dot env
import dotenv from 'dotenv'
dotenv.config()

// Custom Password Specifications
// Username Schema
const usernameSchema = new passwordValidator()
usernameSchema.is().min(3).is().max(18).is().not().spaces()

// Password Schema
const passwordSchema = new passwordValidator()
passwordSchema.is().min(8).is().max(100).has().uppercase().has().lowercase().has().digits().is().not().spaces()

const PORT = process.env.PORT || 5000
const neuronjs = neuron()

// Middleware
neuronjs.use(bodyParser())
neuronjs.use(cors())

// Mongoose Connection
mongoose.connect(process.env.MONGO_URI, { useNewUrlParser: true }, () => console.log("MongoDB Connected"))

// API Routes
neuronjs.POST('/api/auth/signup', async (req, res) => {
    const { username, email, password, passwordConfirmation } = req.body

    // Validation: all fields are filled
    if (!username || !email || !password || !passwordConfirmation) {
        return res.status(400).json({ 
            "error": "true",
            "for": "fields",
            "msg": "fill all the fields"
        })
    }

    // Validation: username is valid
    if (usernameSchema.validate(username, { list: true }).length !== 0) {
        return res.status(400).json({ 
            "error": "true",
            "for": "username",
            "method_fail": usernameSchema.validate(username, { list: true }),
            "msg": "username is invalid"
        })
    }

    // Validation: email is valid
    if (!emailValidator.validate(email)) {
        return res.status(400).json({ 
            "error": "true",
            "for": "email",
            "msg": "email is invalid"
        })
    }

    // Validation: password is valid
    if (passwordSchema.validate(password, { list: true }).length !== 0) {
        return res.status(400).json({ 
            "error": "true",
            "for": "password",
            "method_fail": passwordSchema.validate(password, { list: true }),
            "msg": "password is invalid"
        })
    }

    // Validation: password is confirmed
    if (password !== passwordConfirmation) {
        return res.status(400).json({ 
            "error": "true",
            "for": "confirmation",
            "msg": "confirmation password needs to match password"
        })
    }

    // Check for existing user with email
    const existingUserWithEmail = await User.findOne({ email })
    if (existingUserWithEmail)
        return res.status(400).json({ "error": "true", "msg": "a user already exists with this email" })

    // Check for existing user with username
    const existingUserWithUsername = await User.findOne({ username })
    if (existingUserWithUsername)
        return res.status(400).json({ "error": "true", "msg": "a user already exists with this username" })

    // Generating salt
    const salt = bcrypt.genSalt()
    .then(salt => {
        // Hashing password with bcrypt
        const hashedPassword = bcrypt.hash(password, salt)
        .then(hash => {
            const newUser = new User({
                username,
                email,
                password: hash
            })
            // Saving the user
            newUser.save()
            .then(savedUser => {
                const newPortfolio = new Portfolio({
                    user: savedUser._id,
                    description: "",
                    socialMediaHandles: {
                        github: savedUser.username,
                        dribbble: savedUser.username,
                        twitter: savedUser.username,
                        devto: savedUser.username,
                        linkedin: savedUser.username,
                    }
                })

                // Save the portfolio
                newPortfolio.save()

                // Return the status code and the json
                return res.status(200).json({
                    savedUser
                })
            })
            .catch(err => console.log(err))
        })
        .catch(err => console.log(err))
    })
    .catch(err => console.log(err))
})

neuronjs.POST('/api/auth/login', async (req, res) => {
    try {
        const { username, password } = req.body

        // Validate
        if (!username || !password) {
            return res.status(400).json({ "error": "true", "msg": "fill all the fields", "for": "fields", })
        }

        const user = await User.findOne({ username })
        if (!user) {
            return res.status(400).json({ "error": "true", "msg": "no account is registered with this username", "for": "username" })
        }
    
        // Compare hashed password with plain text password
        const match = await bcrypt.compare(password, user.password)
    
        if (!match) {
            return res.status(400).json({ "error": "true", "msg": "invalid credentials", "for": "password" })
        }
    
        // Create JWT token
        const token = jwt.sign({ id: user._id }, process.env.JWT_SECRET)
        return res.json({ token, user: { "id": user._id, "username": user.username, "emai

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