Does the drive make any clicking or grinding noises? Does it sound different than it did before at all?
If there's physical damage to the platters (likely if the drive makes nasty noises) then your data is probably toast.. unless you want to pay a professional to try and recover the data, which can cost hundreds of dollars.
If the drive doesn't spin up at all, the enclosure might be broken, as sminlal suggests. Try a different one or move it into your computer.
If the drive seems to spin up and isn't obviously damaged, there are various tools available to detect problems and potentially recover data from a hard drive. I've used SpinRite to successfully recover data from a dead notebook drive. You have to buy it, but it might be worth a shot if your data is worth a lot to you. (link:
http://www.grc.com/sr/spinrite.htm )