It's become disengaged from the drive lever or wire guide inside, under the hood. Tricky stuff. If you can open it without disrupting anything, you can observe which things affect the arm and action, and maybe affect a solution. Best way is to study the service manual and see which guides control the arm, and then attempt to get them back in place. There are some plastic parts and if this has suddenly happened, it is always possible that something has become broken. Most of the time, I've been able to reset things and get them working. But if a part is broken, it could be a problem. I don't know the exact model but suspect there's a lot of plastic.
Pioneers are pretty robust, but they do use plastic like most of the Japanese tables. If you can observe how it is supposed to work, you stand a better chance of being able to reset it and restore operation again. In the PL-5. PL-7 series, the arm can disengage from a wire guide and all kinds of weird things can happen, looks like the thing is done for when all it needs is to be reset. you would not know by looking at it, you need the manual to tell you where it should be.