50mm 1.4 - focusing ring stuck
I took mine apart and found one of the cam slots to be bent from an impact. Also one of the cam follower screws was knocked loose and it's threads partially stripped. That was several years ago, and I don't remember how I got it apart, but I have dismantled other Canon EF lenses by removing the lens mount first, and figuring it out from there. Usually the next step was getting access under the rubber coverings on the focus ring. (and the zoom ring on zooms)
In my case, I was going to order parts and have a go at it, but never got around to it. A few months ago I was sending another lens into Canon, Irvine, CA, and decided to include the 50 f/1.4. For Canon's standard fee of under $100 (with return shipping) they replaced the "lens module" or something like that. It looked like new glass came with all the new cams/barrels/followers.
Personally, I think that was a good price for the fix, but it's still a fragile lens with reputation for focus failure at the slightest bump.