I found the solution to fix it but don’t know the ultimate cause of this.
Solution ->
Just go to any of the .ts files like user.ts or app.ts and press ‘CTRL+S’.
Now it will automatically again build and serve your app and once build is finished ,your app will run fine.
You will have to do this each time you write fresh ‘ionic serve’ in terminal.
But not required if automatically build during development cycle on pressing ‘CTRL+S’.