Check out this thread. One of the choices was to reset the reader but this was with a 505. Might be different with the 600.
https://www.mobileread.com/forums/sho...713#post708970
Post # 2 in this thread talks about resetting the 600. You will lose all your books and will have to start from scratch.
https://www.mobileread.com/forums/showthread.php?t=56371
Once you have restored your reader to the factory defaults you will need to authorize the device with the Reader Library software.
http://esupport.sony.com/US/perl/tut...449&mdl=PRS500