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.
Now if you look up at the inside you can see the clips for the black plastic ring inside of the EF mount plate--pop those clips in to remove that inner black disc. This will free the electrical contacts and you can safely remove the EF mount plate completely now without damaging the ribbon cable.
You'll be left with this
Next you can pop the MF/AF focus switch out by getting the top of it with your finger nails
Now you can lift the top ring off the rest of the lens, the one that has 75-300mm printed on it, you'll be left with
Now you can disconnect the 4 ribbon cables by gently pulling the white locking bit of the connectors outward (opposite direction the ribbon cable enters from). Once they are all disconnected you can lift the PCB (circuit board) off and away from the lens.
I was able to fix my lens by simply applying some tension on each ribbon cable one by one while zooming the lens in and out until it freed up. One of the ribbons seems to have gotten caught inside the lens on something. Once it was freed the zoom barrel could move completely in/out so I reassembled my lens.
Careful if you need to disassemble further you should unscrew these two screws next and be very very careful when you lift the metal plate, it has a bunch of gears below and a motor for the auto focus. I removed the motor just to take a peak below and forgot to snap a picture.