help needed with iphone4 appearing dead

[marc64]

please could anyone shed any light on what is wrong with my iphone4?, phone just went dead one day while sending an SMS message, nothing would work tried cleaning the dock pushed both buttons in the hope it would reset and come back alive but nothing, knowing the battery was nearly fully charged i couldn't see the battery was at fault, after a day or two i needed numbers from the phone as i didn't have internet to get them of icloud i connected the phone to a well known iphone data app in the hope that might be able to get into the phones contacts etc, while connecting it to the pc the app didn't show the phone at all so thought it maybe was fried wtv, then after leaving it tethered to my pc for half an hour or so the phones home screen just lit up, just came on as if it had never been off at all, worked faultlessly for a further day and then went to use the phone and noticed it was dead again!, when i tried calling it says its ''not available'' if i try my number, any ideas? what may be wrong and what you think to fix it greatly appreciated,