I use 20x20x20 filters for my heating system. One side of the metal bars(slots) that hold the filter just fell off. How do I re-attach it? Appreciate anyone who knows.

I am assuming that the "return air" grate assembly is broken. It would be unlikely that the hinged side came free (wore out). If the two studs that hold the other side broke or got pushed back, you could just drill a couple of holes and use longer sheet metal screws to hold it in place. If in fact the grate assembly is worn out, a replacement can be had from a sheet metal shop for probably under $50.