Buttons wont work

4 way button works fine, you can navigate around and play stuff, but the back button and the little green MMC button do nothing. Tried resetting with button on bottom and reformating through windows but all content is still there. How do I "blow out" all media and start from scratch? Unit is in perfect condition, might have 2 hours on it since it was bought 4 years ago. Was working fine before, with no cause to stop working.