Same thing once happened on my old Laney.
Firstly, I'm going to cover my arse. Don't mess about if you don't know what you're doing.
Secondly, I'm going to cover my arse again. I have no electrical qualifications, I just happen to have fixed a guitar amp's fan before.
As someone once said in another thread, there are three rules:
1. Make sure it's unplugged.
2. Make sure it's unplugged.
3. Make sure it's unplugged!
The least you can do is to open it up and have a look. You need to start being careful when you've got into the amplifier itself, you'll find some pretty big capacitors which are more than capable of giving you a nasty bite. If there's any risk of you accidentally touching the terminals, short them out using a voltmeter or something.
If you're not a regular tinkerer, be extra careful when disassembling your baby to make a note of which screws went where, what order things came apart in etc. Make drawings if you need to. It shouldn't be toooo complicated, but you might have to take knobs off (mark their positions first) and disconnect speakers (mark polarities), that kind of stuff. Just generally take your time and think about what you're doing.
Knowing amps, it's unlikely that a wire has come loose. If it has, you shouldn't have too much trouble soldering it back on again. But decent amps (especially bass amps, for big, fat, clumsy, bass playing buffoons) such as Hartkes are built to be thrown around stages and dropped down stairs, wires tend not to just come loose. Fans do wear out though, mine was so full of crud that it has just jammed up. A bit of a clear out and a few squirts of compressed air sorted it out, and all was well.
At the end of the day, unless you really spanner it up, you're unlikely to make things worse.
Good luck, let us know how you get on. Incidentally, what city are you in?