Cannot logon after system recovery

See this Apple article:

If your Mac starts up to an Apple logo or progress bar - Apple Support

FYI, here is another Apple article with other startup issues just in case you need it for a different issue in the future:

If your Mac doesn't start up all the way - Apple Support

Also, it is usually a good idea to mention the exact model of your device when asking for assistance online since we may have ideas of likely hardware causes such as a failing Hard Drive. You can get the exact model by clicking the Apple menu and selecting "About This Mac", or if the Mac is not booting, then enter the system serial number on the Apple check coverage page here, but please don't post the serial number on the forum since it is considered personal information:

View Coverage - AppleCare & Warranty

W.W. Webster wrote:

Thanks, but I might not have made myself clear. The progress bar I am referring to is one which appears after I have entered my password. It is this process which does not complete. Thanks again for your response.

Why using "After using System Recovery" to begin with...?

What was the bigger issue you do not say.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"

Genius Bar Reservation and Apple Support Options - Apple

Outside the USA

Contact Apple Support - Apple Support

W.W. Webster wrote:

Thanks, but I might not have made myself clear. The progress bar I am referring to is one which appears after I have entered my password. It is this process which does not complete. Thanks again for your response.

Same thing if Filevault is enabled. Instead of the Apple logo you see the wallpaper & progress bar.

FYI, when macOS 26.x Tahoe was installed it automatically enables Filevault.

W.W. Webster wrote:

How can I now disable Filevault, if that's what I should do, if I can't get into my system? My system is effectively disabled. I'm locked out!

Filevault is not the problem. If you are seeing a progress bar after entering a login password, then Filevault was unlocked and began booting macOS. Follow the instructions in the Apple article I linked previously for the steps to attempt to fix your computer's boot issue.

Also, I just realized the Apple article I linked is missing a very critical troubleshooting step. Try booting into Safe Mode by using the instructions in the following Apple article, then try the instructions in the previous Apple article:

Start up your Mac in safe mode - Apple Support

After upgrading to Tahoe, the system was unusable. Hence System Recovery in an attempt to recover the situation.

This is usually due to issues with third party installed software. Try booting into Safe Mode using the link I provided a moment ago.

If you are able to boot into Safe Mode, then run the third party app EtreCheck and post the complete report here so we can examine it for clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

FYI, before trying to downgrade the OS, you should first try to figure out what is wrong with the upgrade. And the most common reason for problems are compatibility issues with third party software. With older devices, it could be failing hardware as well. It is a huge hassle trying to downgrade an OS, so you may as well try to figure out the current issue first.