COM 4 not found!!

I used my arduino uno on COM 4 before and it worked well, however when I tried to interface my 16*2 LCD to arduino using the example hello world, the following happened:
-The LCD showed light but no words
-The arduino orange light did light up as usual
-I received a message in the arduino compiler that says "COM4 is currently in use"

I unplugged ardunio from the usb port and plugged it in again and clicked upload..the following happened:
-The port didn't appear in tools > serial port menu.
-I received a message from the compiler that says :"COM 4 is not found" !!

what is happening, any help?
I am using windows 7, arduino is detected by the computer, I checked the control panel.

Check which port is currently assigned to the Arduino within your Device Manager. Unplugging the board while the port is "busy" tend to cause the board to change it's port address when re-attached.