When I turn it on I hear the engine humming, but blades barely move, I have to open the vents manually to see this, they don't open. Never worked with one of these before, any help would be appreciated. It is mounted above my 2nd story ceiling in an accessible attic. Have no idea what the effort is to work on or repair these, it appears to be a pretty big blade.

I have 3 thoughts on this. All require you to kill the power (so the power doesn't kill you) before starting.

This is basically what you should see from above.
http://www.fastandbad.com/blog/wp-co.../atticfan1.jpg

There are a few things that could go wrong:

1. Belt is broken/worn
2. Bearings on blade pulley are gummed up
3. Starting capacitor on motor is bad

The first item is easy to determine. I'd probably change it once you've gone to all this trouble anyway.

To test for the second problem, remove the belt by prying it off the large pulley as you slowly turn it (sticking a screwdriver under the belt may help). Once it pops off, turn the large pulley by hand to see if the blade spins easily. If it doesn't, you need to oil and/or clean the bearing for the fan blade. This is probably the least fun solution. Replace the belt by reversing the removal technique.

If both the first items look OK, then the problem may be electrical. There may be a starting capacitor on the motor that has worn out. Take the capacitor (if there is one) to the supply house for a replacement. There is really no good (easy) way to test it.

This isn't really an everyday homeowner's project, but it can be done if you are fairly handy.

Best of luck,
WallyH