You can usually reinsert it without disassembling the flash head. A practical method is to pull the 14mm diffuser partway out, then slide the bounce card back in behind it at a slight angle. Start with one side first, press that side gently against the edge, then work the other side in carefully. That matches others’ experience with similar Godox units such as the V860.

If the card is bent or keeps crimping, an external bounce reflector is a perfectly workable alternative. Small reflectors that attach to the flash head with an elastic band are commonly used for both on-camera and quick off-camera “run-and-gun” shooting, even by people whose built-in cards are still intact.

So: try reinserting it first; if it’s damaged or frustrating to fit, an accessory bounce card is a simple backup solution.