Hi I recently detected my Apple Watch 5 ECG does not detect my finger. Please help

There are 2 things that fix about 98% of watch problems;

1.Force restart

Press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

This fixes a surprising number of issues.

2.Unpair and re-pair and setup as new.

This returns the watch to the way it was when you bought it brand new.

You can Unpair and re-pair and restore the watch from a backup but the problem with this is that whatever software glitch you had could be reintroduced from the backup you made when the watch was faulty,therefore leaving you no further forward.