My MacBook showed a message which said I had to start my laptop. It wouldn't let me do anything else. Now I'm not able to log in to my administrator account anymore. I can use my guest account though.
Try restarting in Safe mode (hold down the shift key at Startup). This does one pass of "Repair Disk", loads the minimum set of extensions, and gives you a login screen.
Then see if you can login and do typical things. Later, you should restart "regular" and see if your problems have gone away.
OS X Lion Recovery Utility
Lion Recovery Mode
Recovery options for Macs running OS X Lion MacFixIt - CNET Reviews
Retrieving data ...