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I'm assuming a Volvo V70/S60/S80 that this happened on. There is a small plastic piece in the seatbelt that looks like a button. This prevents the seatbelt from retracting too far into the winding mechanism. If the button pops off the belt, the seat belt retracts too far into the winder and makes it so there is no slack in the belt. The seatbelts have a system that prevents them from releasing if they are tilted too far off of vertical. It's a safety system that keeps the seatbelts from letting go in case of a rollover accident. This gets triggered when the seat if folded down. With the button gone, it sticks the seat in the folded position.

The best way to get them loose again, is to pull the side bolster on the outside of the seat. Under it you should be able to get to the body attach point for the belt. If you remove that bolt, you can get the seatback to tilt up again. Once the seat is up and latched, the belt will release. You'll need to get the two little plastic pieces that go in the belt to fix it from the dealer. One of them pokes through the belt and the other goes over the pin. There will be a small hole in the belt where the old one was. Then you use a hot tool to melt the pin. It's a pretty inexpensive fix even if the dealer does it.

Hope that helps!