If it works for a few minutes before dying, then it probably works fine and the heat is just interfering with the tasks. Restoring the heat sink should return it to normal.

If it doesn't work at all, then something melted and it will never work again. You can reattach the heatsink and see if that works (since it's cheap to do anyways), but you're probably going to be disappointed. You will have to replace the entire motherboard if that's the case.