there is no real explaination to why that could happen unless u were online when u left it sit while u did something else and some worthless detsructive hacker hacked into ur comp and blocked u out of it and left a trojan that disables ur logon process otherwise ur windows is missing the main file to login to windows ... did u try deleting the hd and starting with a brand new installation of windows and not just using an upgrade version of windows... its best if u delete the partition, add a new partition, and make a new bootable install of windows with all the files it needs for everything then download and install sp2 if ur on xp or anything else... its not often that a problem like that can occur but is very possible... if it happens again try the way i explained if u have a full install disk of windows...