My app is crashing during JWT Authentication

Question

I am new to MERN stack development and trying the JWT authentication. During this I am facing that my server is not working they crashed every time.

Yesterday It was working but today It's not working. Now they are showing this error.

This is error showing on console

MissingSchemaError: Schema hasn't been registered for model "users".

Here is my Schema:

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const UserSchema = new Schema({
    name: {
        type: String,
        required: true
    },
    email: {
        type: String,
        required: true
    },
    password: {
        type: String,
        required: true
    },
    avatar: {
        type: String
    },
    date: {
        type: Date,
        default: Date.now
    }
});

const User = mongoose.model('users', UserSchema);

module.exports = User;

This code is in the routes folder - user.js file

// user.js

const express = require('express');
const router = express.Router();
const gravatar = require('gravatar');
const bcrypt = require('bcryptjs');
const jwt = require('jsonwebtoken');
const passport = require ('passport');
const validateRegisterInput = require('../validation/register');
const validateLoginInput = require('../validation/login');

const User = require('../models/User');


//Register Post Router
router.post('/register', function(req, res){
    const {errors, isValid} = validateRegisterInput(req.body);

    if(!isValid){
        return res.status(400).json(errors);
    }

    User.findOne({
        email: req.body.email
    }).then(user => {
        if(user){
            return res.status(400).json({
                email: 'Email already exists'
            });
        }
        else {
            const avatar = gravatar.url(req.body.email, {
                s: '200',
                r: 'pg',
                d: 'mm'
            });
            const newUser = new User({
                name: req.body.name,
                email: req.body.email,
                password: req.body.password,
                avatar
            });

            bcrypt.genSalt(10, (err, salt) => {
                if(err) 
                console.error('There was an error', err);
                else{
                    bcrypt.hash(newUser.password, salt, (err, hash) => {
                        if(err)
                        console.error('There was an error', err);
                        else{
                            newUser.password = hash;
                            newUser
                            .save()
                            .then(user => {
                                res.json(user)
                            });
                        }
                    });
                }
            });
        }
    });
});

//Login Post Router
router.post('/login', (req, ress) => {
    const {errors, isValid} = validateLoginInput(req.body);
    if(isValid){
        return res.status(400).json(errors);
    }
    const email = req.body.email;
    const password = req.body.password;

    User.findOne({email})
        .then(user => {
            if(!user) {
                errors.email = 'User not found'
                return res.status(404).json(errors);
            }
            bcrypt.compare(password, user.password)
                .then(isMatch => {
                    if(isMatch){
                        const payload = {
                            id: user.id,
                            name: user.name,
                            avatar: user.avatar
                        }
                        jwt.sign(payload, 'secret', {
                            expiresIn: 3600
                        }, (err, token) => {
                            if(err)
                            console.error('There is some error in token', err);
                            else{
                                res.json({
                                    success: true,
                                    token: `Bearer ${token}`
                                });
                            }
                        });
                    }
                    else{
                        errors.password = 'Incorrect Password';
                        return
                        res.status(400).json(errors);
                    }
                });
        });
});

router.get('/me', passport.authenticate('jwt', {session: false}), (req, res) => {
    return res.json({
        id: req.user.id,
        name: req.user.name,
        email: req.user.email
    });
});

module.exports = router;