Am tooo facing the same issue with my 720... its been 3 times now... i can recognize that the system is up and running but Display off and wont start whatever i do.. untill i restart using volume and power keys or... shut down the system blind folded assuming the "slide down to turn off" appears....
I am not using many third party apps... infact latest when i did it... i guess only IE and Music player were running... Wi-fi and blue tooth off....Mobile data was on...
Nokia must Just gotta solve this soon... Its a blind trust i have on Nokia, which made me value 720 ahead of the numerous competetion.. expecting a patch soon...