To be honest these are the solutions that may or may not work.
First thing I would try is rebooting the phone by holding the sleep/wake button and home button for 15 seconds until you see the white apple logo.
If that doesn't work I would restore from backup. If that doesn't work I would restore as new. If all else fails take it to your closest apple store they may replace the phone for a working one..
Thank u for your help but I already did restore to my iphone many times but this did not prevent this problem from occuring again..oh this iphone is the first product i bought from Apple company after many years with Nokia and i have to give it back to apple store to fix it as till now this iphone give me only very bad impression about quality of Apple's products as it full of bugs including slow unstable wifi connection , strange black screen and antenna problems and i really hope that Apple take care from hardware problems in its upcoming iDevices
Message was edited by: tad88
my iphone4 had a blank screen and had to the hard reset. i thought i'd attempt to sync the phone first. iphoto was able to recognize the phone and was able to import content i had taken earlier in the day. iTunes didn't do anything at all besides showing me the spinning ballon.
i am not sure why my screen went blank either, it was sitting on a table for a few hours. there was decent battery life as well. i hope it doesn't occur again.