Isnt "the one attached to the motherboard" the (north?)-bridge fan?
There are usually 2 fans in that area:
1 on the cpu (a big one on top of a piece of metal, about 15cm thick)
1 on the bridge (very small fan, no metal heat sink attached)
Which of those isnt spinning? The small one isnt important on some motherboards.
On mine for example I can take it off completly and the bridge doesnt even warm up.
If the big one isnt spinning -> Code Red!
-> do as Claies said
Oh, and better leave the comp switched off until you get a new one.