Arduino serial monitor stopped working

I have been running Arduino IDE on a laptop for some time debugging my sketch involving a TFT LCD.

But out of the blue serial monitor has stopped working.

I tried restarting the laptop - same problem.

I tried ruuning an example sketch from Adafruit_TFTLCD - the sketch runs but no output in serial monitor despite the fact that I changed Serial.begin(...) in setup() to the baud rate I am using.

I tried running my sketch without serial monitor open and it works fine.

But as soon as I open serial monitor my sketch stops working an I get a white screen.

Has any body encountered this problem?

If so how do you fix it?

Re-install Arduino IDE?