Happens to me regularly with my Z240s. I switch between S-TTL and Full and after a while it usually starts working again. Sometimes turning it off for five minutes helps as well, or pulling the pre-flash button out. I keep an eye on the red light and charging noise to check whether I'm getting a full dump out of the strobe.
Beware of repeated shots at full power in warm water as the Z240s overheat and (as well as browning the tubes) it cooks the internal electronics.
- Grantjpthomas likes this