i have a pioneer elite pd-f107 100 disk changer?

At the age point of this player, it's common to have belts stretch and deteriorate. Since the player is setup in groups - the laser system is hung up at the first group of discs and can't move into the 2nd through 4th group.

Very possible DIY, just be sure to unplug it while opening up the cabinet. Oil is the worst enemy of a belt driven system, so keep your fingers washed often. With extreme care, plug it in and watch the actual transport (laser/drive) move and pay attention to what is being hung up. If it's a belt slipping - that's the problem. You can wash a belt in detergent and as hot of water as you can stand, and it might work temporarily.

If a new belt is needed, you need to measure it exactly for thickness and length, keeping in mind that 1) you need to double the belt length when stretched and 2) go for slightly smaller due to stretching. For one belt, eBay may be your friend here. The companies I purchase belts from have high enough shipping cost that between 1 belt and S/H, eBay ends up being cheaper.