Please help, phone wont switch on

My iphone 3GS switched itself off and it won't switch back on again. I tried pressing the start/sleep button, that didn't work, then I pressed the start/sleep button and the home button together and that didn't work either. I plugged it into itunes and it doesn't know it's there. The last thing I did when it switched off was take out the earphone's. Please help, it's been 2 hour's and I'm starting to get iphone withdrawl's!!