Restore

So the calamity has happened: your conclusion is the SD card has failed in the Pi and you need to get it up and running again ASAP
Fear not! Here is what you can do:

Either reuse the possibly faulty SD card or preferably use a spare/new one and proceed to flash a fresh image from your latest backup

You'll need Balena Etcher or other imaging utility to flash the backup.img to the SD card (mine shows as a USB device with two partitions mapped to D: & E:)

Don't be alarmed if you see a few popups during and after using Etcher. It's perfectly normal as the partitions are created (and Windows is keen to format them, especially as one of them is non-FAT ext4 Linux partition)

But make sure you always select Cancel and not Format disk !

After flashing is complete, before you put the SD in the Pi and boot it, you might want to check a couple of vital files exist in the FAT32 boot primary partition because without them you may not be able to remotely access your restored Pi via SSL.

To do this check you would need need to reinsert the SD for Windows benefit (certainly if you used Balena Etcher, because it automatically does an eject).

Hopefully these files will exist and no action will be needed to recover them:

ssl
wpa_supplicant.conf

I keep a copy of these files in my \\NAS\share just in case they are needed.
The first enables ssl so that you can PuTTY to your Pi after it starts up
The second is needed to hold your network details, quite important if your rely upon WiFi connectivity!

And finally

After you've booted your Pi from the fresh SD you'll need to expand the Linux partition.

Use the "7 Advanced Options", "A1 Expand Filesystem" option from raspi-config

After a final reboot your PI will be restored