When a card becomes unusable, you typically have two "tool" paths depending on your hardware:

Hold the switch down until the STOP LED lights up (approx. 9 seconds).

The Siemens MMC Card Format Tool is a lightweight, standalone utility provided (unofficially but widely supported) by Siemens to restore MMC cards to a factory-ready state. It is different from the "Transfer" tool inside STEP 7 software.

If you have a PG (Field PG M5/M6, etc.):