Shut Down Mac, and Now Requires Password?

Question

My Mac hadn't been shut down in a couple weeks (it was running just fine so I kind of forgot to shut it down and give it a brief reprieve).  When I did shut it down by hitting the button- because it froze on me and I couldn't do it any other way- it shut down normally.  Now, however, it appears that my password settings are reset and it requires a password every time I log in. 
I know how to change the settings back, and I know my password.  But, I'm wondering why my Mac would suddenly switch to requiring a password without me initiating the change?
Thoughts would be appreciated!

Answer 1

Well lets clean you out first.
Download and run ALL of the free OnyX initial checks, cleaning and maintainence aspects and reboot at the end (you can cancel the in between reboots)
http://www.macupdate.com/app/mac/11582/onyx
Scroll down of your OS X version is earlier. Please mention your OS X version, your Mac's model and if OnyX gives any warnings on the initial checks, it's important.
If you get a warning, please quit OnyX and backup your files off the computer as soon as possible and disconnect this drive.
You may have to hold c and boot off the 10.5 or 10.6 disks and run Disk Utility, or hold command r boot and run Disk Utility from the Lion Recovery Partition on the drive/OS X partition and Repair Disk, Repair Permisisons.
If Repair Disk gives any errors, please state so. Repair Permissions always gives a long list, it's ok.
After this is done, also run OnyX > Verify >Preferences and copy down any corrupt plist (preference) files.
Mention those here and I'll tell you where to find them to remove them.
Once all your problems are cleared, there is a System Preferneces > Accounts > Log in items where you can change things, see if you can change your desktop, screensaver etc, it these setting don't remain upon reboot, that's a sign of a corrupt system preferences plist file.