You still using ASUS A8N-E? If so, this is a COMMON problem. Mine has died several times.
First, see if your fan can be removed without removing the mobo. The old revision fan requires you to remove the pins from underneath, but the new one can be taken off from the top, without moving the mobo.
If you have the old revision fan, call ASUS, go straight to their RMA department (skip customer service, they'll try to charge you), tell them your fan died, and they'll send you the new revision.
Now, if you already have the old revision, you're straight. Just remove the fan. Blow it out well with canned air (these fans are weak, so any resistance from dust clogs them up). Hit the fan's center with WD-40. Blow it off again with canned air. Place back in your mobo and enjoy. Preferably, scrape off the base and add some new thermal compound to it.
It's ok if the fan isn't 100% dry. WD-40 isn't conductive, so if a few drops get on your mobo, no harm no foul