I am a novice Marklin collector so please excuse the question. I have a digital layout that uses two handheld controllers, one is assigned to the locos and the other handles turnouts and uncouplers. I have 4 trains. The layout is about 3 years old. Everything has been running fine, but suddenly the locos will only move less than an inch, then it appears that they lose electrical contact with the track, they stop moving and lights go off. If I move them a fraction of an inch, they will run again for less than an inch. All 4 locos are behaving this way. I have tried cleaning the rails with rubbing alcohol and a cloth and a lot of grease and dirt comes off and there is some small improvement but not much. I tried deleting then trains from the controller and then adding them back to the track one at a time. The problem remains the same. Does anybody know the problem, can someone please help me. Thank you.

There may be several reasons.
Check there is no rust on the centre rail. Outer rails on C track and modern K track cannot rust, but centre rail still does.

To clean the rail and stud contacts is a good advice.
But i wonder if you have suddenly power failure?
Did you tested if change turnouts or uncouples work?