OK. Updated to 17.4 and all seems to be good. Big thankyou to those who sorted this out. I don’t know the first thing about coding so totally stuck without those who do !!
Glad to hear I’m not alone with this problem of Gutenberg 17.3.0 simply not functioning for me. I’m running Faith theme (church website) and WordPress 6.4.2 – everything up to date. Everything worked perfectly until the 17.3 update. If I could go back to the previous version of Gutenberg I’d be a happy man! Hopefully a fix comes soon.