As I recall (it has been years since I did it), the slide is not fastened down in any way - it is held down by gravity. With the slide pushed all the way in, you can lift the forward edge and peek underneath, but it won't lift very far. ISTR that I moved the slide all the way out, then lifted the front and rear edges, which would go up a lot in that position, and propped them up. That gave me ready access to the underside. I could see and manipulate the rollers and the wooden blocks, and in my case I added another roller in the center. I believe you will find that one of the blocks has moved or deformed, and can be refastened or replaced without too much trouble. Just be sure that the slide is securely blocked up. It is very heavy, and if it suddenly drops, it won't be fun.
To prevent it from happening again, when you pull the slide out, don't let it slam into the blocks.