No boot beyond fuzzy white screen

MY 2 year old S3 has been freezing lately. I've got round this by taking battery out and restarting. Last time i did this I got a fuzzy screen. buttons are unresponsive although the power button does get it to this stage. When i plug it into the computer, the computer recognizes the phone name but wont let me get at files. I get the same thing if i plug it into the computer when its turned off and if i plug it into the computer without the battery in, so i don't think the battery is the problem. If i got a new screen would i be able to access files or is it likely to be more broken than that? I'm suspecting a motherboard problem...

Thanks for any suggestions