I'm experiencing a problem I can't solve. This line causes the serial input (Serial over USB) to stop working. Output is fine. Commenting out the line solves the problem.
// initializing LCD panel and setting interface pins
LiquidCrystal lcd(12, 11, 5, 4, 3, 2) ;
You started off saying you couldn't solve it, then you solved it.
In case commenting out the initializer for the lcd is not a good solution, you might want to post all of your code, say what Arduino you have, and mention what operating system and version of the IDE you are using.