Hi jenveitch and Welcome to Apple Discussions,
I had a message on my iMac that my startup disk was full and then it wouldn't run any applications so I decided to restart the machine.
First try starting it in target mode:
http://docs.info.apple.com/article.html?artnum=58583
If it will go into target mode you can save your stuff via FireWire to another Mac as per that site.
When everything is saved "get info" about that hard drive and see if it really is full. If not the OS might be corrupted. If it is full try dumping as much as you can and try restarting. If it won't restart and in either case it might require a reinstall of the OS.
Richard
Message was edited by: spudnuty