If your Alt-Tab shortcut suddenly stopped working, there could be a few different reasons why this is happening.
One possibility is that another program or process is using this shortcut and preventing it from working properly. You can try closing other programs or restarting your computer to see if that helps fix the issue.
Another possibility is that your keyboard settings may have been changed or there could be an issue with your keyboard drivers. You can try going into your keyboard settings and checking the shortcut keys to see if Alt-Tab is still programmed to switch between open windows. If not, you can try resetting the settings or reinstalling the keyboard drivers to see if that fixes the issue.
If neither of these solutions work, there may be a more technical issue with your operating system or software. In that case, you may need to consult with a professional technician or IT support team to help diagnose and resolve the issue.