I've just turned off the hot water at the stopcock in the airing
cupboard (one of those red-wheel jobs.

Something has broken, and it won't turn back on again.

I've removed the wheel, and the centre stem is just turning endlessly
without pulling the shutter (don't know the right terms!) back and
openening the pipe.

No hot water tonight is going to be popular. Is there a way forward
without draining the whole system down?

TIA

Pete