I have the same thing from time to time, and also the pilot light seems to fail to ignite. I had it recently serviced and told the engineer who was unconcerned, said it had something to do with air.
When it happens, usually if i switch on the heating and run the hot water, it really sounds like theres someone in there trying to get out 😆 and the first time i thought is this thing going to explode 😯
I switch it off at the mains, keep the heating off then run the hot tap. The boiler lights and the pressure shoots up, but no hot water comes out. The temp gauge goes to max(9) but then slowly comes down. I just spend a few minuted fiddling with it, tap on, tap off, see it it lights, then switch the heating back on and it seems to sort itself.
It has in the last 6 months happened about 3 times, and each time a bit of fiddling, waiting, system on system off and it rights itself.
The fact the engineer was unconcerned made me thing its maybe to do with the weather or air pressure or something, but nothing to actually worry about. Last time was about a fortnight ago, and the same fiddling about brings it back on line.