Derek Geldard wrote:

I don't know how the connections are made underneath the plastic skin
of a "sealed for life" battery nowadays, probably you might as well
replace it if the same thing happens, not much alternative.

I had two batteries fail in the manner described - total sudden
failure - within a week of each other.

I was told that on modern batteries the top section, including the
connector posts, is a push-fit on the rest of the battery. Sometimes
these connections just fail and, being internal, are uncurable other
than by battery replacement.