My zoom lens barrel was stuck and wouldn't zoom out--it would stick at 100mm and wouldn't move further in.
Start by removing the 4 screws at the back of the lens on the EF mount.
Then remove the 3 tiny screws inside of the EF mount, 2 hold the electric contacts inplace.
Now lift the EF mount plate up, you may have to rock it side to side gently, you won't be able to lift it very far due to a ribbon cable so be careful.
Dust behind your front lens element? Not to worry--disassembly is easy:
Remove the 3 black phillips screws around the front edge:
You can now lift that front ring off the rest of the lens.
Don't bother trying to unscrew these next 3 phillips head "screws" they are pins that you'll need to remove with some small needle nose pliers. Pull them straight out, they come out very easy.
Now carefully tip the lens over being careful not to let the front lens element fall out, if it doesn't fall out easily gentle tap and shake the lens a bit and it should fall right out.
This is what the inside of the lens looks like
Now you can clean the dust off the back of the front lens element, reassemble by reversing the disassembly process.