You could try just deleting some files to see if that makes any difference in case Hemmi's suggestion about it being a particular book/file that is causing the problem.
If that doesn't work, it sounds like you've done a soft reset (press the Reset hole with a small pin and after 5 seconds sliding the power switch to restart the reader), so the next step is to try a hard reset. This would wipe everything off your reader though, pretty much restores it to factory settings. So you'd need to make sure you have a back-up of your books first, and be aware you will need to reauthorise it with Adobe.
See the MobileRead Wiki for info on resetting your 650:
https://wiki.mobileread.com/wiki/Sony...nd_Reset_Guide