My well casing/cap froze over a little a couple of months ago, despite having a hard cover (one of those fake rocks).

If you have a cover, I'd buy a heat lamp and run an extension cord over to the well and just leave it on. If you can't get a heat lamp in time, what I did was put a tarp and some moving blankets over my well and it worked out just fine. It didn't freeze again during the next cold snap.

You can also buy one of the insulated well pump covers - it's like an insulated ziploc bag that goes over the pump and does a pretty okay job.

Nothing will beat the heat lamp, though.