COM3 not found

I installed Arduino ide long back. it was working fine. i could upload codes to it. but then suddenly it stopped working.
when i upload a code to it i get the error that COM3(the port to which i have connected my arduino) not found. can anyone please help me with this? :~

Which pc OS? Which Arduino? Does COM port appears on port list?

i have a win8.1 64 bit pc. i have atmega328. that serial port option is disabled. :confused: