essentially, something happened which prevented the machine from loading the registry keys which make up your personal profile
on the reboot it recovered from this and loaded up from a backup copy.
the big question is, why did this happen? Commonest reasons for a registry failing to load properly are
faulty / loose RAM memory
hard drive corruption (e.g following bad shutdown)
failing hard drive
virus / trojan / rootkit
so, reseat / check the RAM
run chkdsk C: /r from a command prompt to scan the drive for errors
update your security software, do a full scan and also scan with Hitman Pro and Combofix