A photo would be of great help. Most of the time there is a steel plate or angle iron used as a lintel that supports the bricks above the opening.Bricks above the opening have fallen out. I don't know how to get them back up there. Do I put some sort of bracing across the top of the opening then build up? Do I mortar or use some sort of adhesive?
If the firebox is collapsing, there is a strong chance the chimney has major flaws also.
It looks like Uncle Joe or Cousin Moe did some older “repairs” on those bricks that caved in.
And might not have paid any attention to what they couldn’t see from their recliner.
So you might be missing some bricks higher up, or at least have open mortar joints, letting flue gases or sparks or fire escape into your house structure.