Computer Trouble

Ok, so, apparently my computer decided to do something tonight.
I went out for a bit, shut my computer off, it sat around for a good 6 hours before I got back and turned it on.
I see the HP logo, then get an error "system fan is not working; shut down computer to avoid damage" all that stuff.
So, I turn it off, give it a couple minutes and start it again, same error.
This time, I shut it off, pop open the side panel and turn on the computer. All the fans are running it seems.
So, I continue on and hit F2 to boot up the system to come on here and ask for some advice and look around online for any similar problems.
Edit: I forgot to add that when I shut down the computer completely to write down the error message. Upon starting the computer back up, I received no error, and the system booted up normally.
So, anyone got a suggestion?
I went out for a bit, shut my computer off, it sat around for a good 6 hours before I got back and turned it on.
I see the HP logo, then get an error "system fan is not working; shut down computer to avoid damage" all that stuff.
So, I turn it off, give it a couple minutes and start it again, same error.
This time, I shut it off, pop open the side panel and turn on the computer. All the fans are running it seems.
So, I continue on and hit F2 to boot up the system to come on here and ask for some advice and look around online for any similar problems.
Edit: I forgot to add that when I shut down the computer completely to write down the error message. Upon starting the computer back up, I received no error, and the system booted up normally.
So, anyone got a suggestion?