Cooling Fan Problems

I would make sure it is getting power (not sure if it's fused or not) using a multimeter. Then I would look into the thermostat that controls the fan. If both of those are good, the fan motor might be the culprit.

Anyone else?

I would start by doing the same. Find the plug for the fan and unplug it. Test to see if there is any power at that point. You will most likely need to run the bike a little to make the fan temperature sensor kick in.

Do you have a service manual.

I would either do what Battlegun and GrizzWiz suggested or just run a 12 volt jumper wire and a ground wire to it to see if it would start to spin up. If it does, you know the problem is elswhere and can begin checking for a fuse, a temperature sensor or just a simple break in the line.

Certainly there's a fuse for the fan. A quick check in the owner's manual may reveal it's location so you could see if it's blown.

Buster

To add to the list of possibles, if you run your quad in the mud or water you may check that the connections are not corroded. Additionally, if you do subject your quad to these conditions an application of dielectric grease could prevent this problem in the future. The tubes of dielectric grease are not that expensive and can be picked up any where auto parts are sold.