Alarm no longer switches off when tapping screen - what has changed?

Unless the screen developed a fault it could be just a temporary glitch. Does the screen work when you tap it for other functions? If yes, does charging the watch change anything? It goes through a sort of soft reset every time you charge it. You can also try to connect and sync it with your computer through the Garmin Express app (you have to download and install it on your PC).

If none of that works, wait until the battery discharges almost completely, to the point when only the mechanical hands work but the display stays off, the Bluetooth sync does not operate and the green and red LED in the back for heart rate and pulse Ox measurement do not lit up anymore.

Only then give it a full charge, you should see the Garmin triangle logo appear for as few seconds on the display once you attach the charger and then the charge continue as usual.

If even that does not help, it is worth to reset the watch. You do that going in the Setting -> Reset menu through the watch screen. You will have two choices, "Reset Default Settings" or "Delete Data and Reset Settings". I prefer the latter, for a more thorough clean-up. IMPORTANT: Make sure you do a manual sync with the Garmin Connect app just before you do the reset, as all of the data in the watch will be wiped. That way you won't lose anything, it will be saved under your Garmin account. Before doing such reset I also remove the watch from the Garmin Connect app on my phone and from the list of paired bluetooth devices (in the phone).

You will lose all of the customization you may have made to the watch, like the appearance and color of the screen, the activities you want to be able to select on the screen, the alarms, etc. You will also have to re-pair the watch with the phone, as you did the first time you activated it after purchase.

It sounds more complicated than it is, just a bit time consuming if nothing else.