If there’s one thing you don’t want to happen with any Mac with a T2 chip it’s a problem with a firmware update. If you only have the one Mac, contact Apple Support, as the supported methods require a second Mac (which doesn’t need its own T2 chip) and the current release of Apple Configurator 2.

If you do have a second Mac with which to fix the sick one, study this document carefully, as it explains exactly what to do.

Apple draws a distinction between reviving and restoring the firmware. This is very important: if you can revive rather than restore, no data will be erased on that Mac. If you must restore the firmware, that restores firmware on volumes on internal SSD storage, which destroys all existing data on your internal SSD.

Reviving the firmware is usually needed if something, such as a mains power failure, interrupts a firmware update. It leaves the T2 chip unresponsive. If revival is successful, then you should be able to continue using that Mac very quickly.

If revival is unsuccessful, or Internet recoveryOS is unsuccessful, or you can’t start the Mac from its startup volume or the recoveryOS, then the only option is to attempt to restore the firmware. On completion, you’ll then need to re-install macOS and restore the contents of your internal SSD.

Thanks to Mr. Macintosh @ClassicII_MrMac for this information.