This happened to mine, but stuck in the closed position
so I wasn't getting any heating. Some lubrication and
manual operation opening and closing should free it up.
err, what you will find after a bit of manual operation is that it will
break and leak onto the distributor, then you have to buy a new one.
I spoke to a chap at an MG parts place when I bought my new valve at the
end of summer last year. He said he sells a lot of them at that time of
year, because people turn them off then don't touch them until it gets cold
again, then they break due to under use.