Hi Prudence,
There are a couple of reasons why your fingerprint sensor may have quit working, but without looking at the phone it's hard to say exactly what the problem is. I'm thinking it could either be a loose connection inside the phone, or possibly the sensor has failed and needs to be replaced.
Since the power button and the fingerprint sensor are built into the same part, it might help to know if the power button is working or not.
Either way, if it was my phone, my first idea would be to open up the rear cover and check the connector. I wasn't able to find a guide for your phone specifically, but I did locate one for the Galaxy A12 Nacho; I'm hoping yours isn't significantly different from that one.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a12-nacho-1361/sensor-huella-y-bot-n-de-encendido-12290|Manuals / Samsung Galaxy A12 Nacho / Fingerprint sensor and power button|new_window=true]
Anyway, take a look at Step 1 of the guide that shows how to remove the rear panel; that's the connector you want to check. Verify that it's firmly plugged in and hasn't worked its way loose over time. If that doesn't help, try unplugging it and then clean both the cable and the connector on the board with some 90% or higher concentration of isopropyl (rubbing) alcohol. While you're at it, take a magnifying glass and check both sides for any bent or broken pins; sometimes bent pins can be straightened with some fine tip tweezers, but broken pins will probably require you to replace parts. Check that and let us know what you find.
So if you get this far and still can't get it working, then it may well be time to try replacing the power button/fingerprint sensor. Here's one example from Amazon, but you'll need to check your model number to see if it's a match; the device page for your phone calls it model SM-125, but the Amazon page says their button is for the SM-125F. I don't know if they're compatible or not, but you should be able to find one specific to your phone if it's not a SM-125F.
[link|https://www.amazon.com/Samsung-Galaxy-SM-A125F-Replacement-Connector/dp/B08Z45VZMF|Amazon.com: fonefunshop for Samsung Galaxy A12 SM-A125F Replacement Power Flex Connector : Cell Phones & Accessories|new_window=true]
Good luck; let us know what you find!