serial port disappeared and can't upload code

Hi guys, this problem I'm experiencing with my arduino uno board has got me by the short and curlies. I purchased a gprs Shield from seeed, the v2 one, when I finally got it working I use a sample program found on the Wikipedia. The board looks like the code is running because all the lights are one and the signal light is flashing but when I run the serial port monitor the program says the port doesn't exist. Now the port has disappeared and I can't upload code or even pick up the port I was using in device manager. Please help someone, anyone.....