The front element is held by the decorator ring, that's the ring with the f/l & f stop info on it. I use a rubber stopper held at an angle to unscrew them. Just hold the stopper at an angle & unscrew. Sometimes you will have to do this at different points around the edge to get it started.
The 50 has been dropped or bumped & the helical has jumped a thread it was very common. Almost the entire lens has to be taken apart. About the only thing not disassembled are the aperture blades.
If you do this take notes & mark some reference points with a scribe so you can get the timing back.
Remove the lens mount, 3-4 screws, aperture ring, slides off- maybe a loose ball bearing, There is a guide usually with two screws in it visible from the back. If you remove it the focusing ring can be unscrewed. BUT
if it's jumped a thread it's going to be very tight. I used to use a piece of wood or toothbrush handle & set it on the back of the ring & tap it with a (small) hammer to force it forward. If it won't go forward, you have to remove the decorator ring & force it backward. It's best if you don't remove the focusing ring from the barrel as you have eight threads inside & outside & getting everything back in place is a PITA because the lens will only focus correctly with one pair.
