In addition to everything that sigmadelta wrote.
Even if you have a backup STEP7 project, it may be wise to try and fetch a backup from the defective CPU. It can happen that the backup is not uptodate.
This is the order which I would proceed:
1. Attempt to upload the program to a new backup.
Create a new empty project in STEP7, then use Upload to PG. If succesful compare the old backup with the uploaded program. If there are differences, investigate these and decide what to do about them, incorporate them in the previous backup or discard them.
Load the updated backup program in a new CPU with a new MMC card.
2. If 1 is not succesful, install the old MMC card in a new CPU.
If the CPU goes to run, then similar to step 1 perform an upload and do a compare with the old backup etc.
3. If 1 and 2 are not succesful, load the old backup program into a new CPU with a new MMC card.
After 1, 2 or 3, if the CPU goes to run, check that the machine works OK.