Latest version of OS installed overnight on MacBook Air. Computer now completely inoperative.

Shutdown by whatever means required. Connect to charger and disconnect any external drives. Restart and immediately Hold the SHIFT. This will do a Repair Disk, clear cache files and only load Apple Software, extensions and Fonts. It load slowly - normal. This may or may not allow the process to complete - hopefully successfully and presented with Setup Assist of Big Sur.

If I may, I have a question for you. Did you do the download via Wi-Fi? If so, that is likely to be the problem. An OS installation file is a very large download and Wi-Fi is simply too slow and sometimes there are interruptions which cause the file to become corrupted. Then it either won't load, or if it does - problems. I recommend that for these OS updates you use a direct connect to your modem with an Ethernet cable. That will help. The download should only take about an hour or so, give or take. Then the installation usually takes another 45 minutes or so, more or less, with several automatic restarts. Good luck.