The solution to fix biometrics is to clear the cache in the samsung pass app. It worked for me with a S9 FE. I could not get recovery mode to work using a USBC cable with a computer as recommended.
Update: Clearing the Samsung Pass cache worked once but then the problem with the fingerprint sensor returned. There is a system app called Biometrics and a few others that are involved. An app called 1-tap cleaner will clear all app caches. It defaults to cleaning large apps. You can set it to do them all regardless of size which I did. After running through 274 apps the fingerprint sensor is working again. It takes a while to do them all (the app allows you to automate that).
Final update: Have had to run 1tap Cleaner several times as the fingerprint sensor problem kept quickly reoccurring. Decided to add a new fingerprint and delete the old. That seems to have stabilized things.
OK, so I've posted some solutions that seem to work only sometimes so I decided to do a factory reset. That went well and now when the fingerprint sensor stops working and says to restart I do and that now fixes the sensor. It doesn't fix it permanently but whenever it stops working and I do a restart it resumes working. Perhaps this issue is being caused by an update gone wrong. But the factory reset seems to have restored normal functioning.
