Photoshop fails to open

​@Gil - Solihull, do you mean to say that you just “upgraded” to the v27.8.0 release of Photoshop? Because you used the word "downgraded," and it seems confusing. Try rolling back to an earlier version. From the Creative Cloud Desktop app, click on the three-dot menu under Photoshop > Other versions, and reinstall v27.7. Make sure your GPU driver is up-to-date. Check out what Derek suggests or wait to upgrade to the next version until you have more time to figure out what might be holding it up from loading. You can also cut-and-paste your system info with us here (found under Help > System Info), and we’ll see if we spot anything that might be causing your problems.

How much RAM and spare hard-disk capacity do you have?

Maybe uninstall Photoshop and reinstall it.
Try reseting the Photoshop Preferences, found under the General tab in Preferences.