Been trying to find the stopcock in my house and came across this tap in the downstairs toilet, does it look like the stopcock? If it is it doesn't work (i.e. it turns but doesn't stop the flow of water. Editor note: from a comment on an answer)
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Yes, that is almost certainly your stopcock, they very commonly seize up, especially if they are opened 'hard'. You could try tapping firmly on the end of the stem, whilst trying to turn it, to get it to release, but it's likely to be jammed solid.
It looks like you're in the UK, so you should, hopefully have a stopcock out in the street. It'll be with the meter if you have one.
PS - if you do manage to free it off, leave it a half-turn closed, to reduce the chance of seizing again.
I agree with SiHa. Also, notice how the body of the valve has a hex nut formed into it. This means you can rebuild it. If you find a valve upstream to stop flow, simply remove that entire assembly and take it to a good hardware store or plumbing supply shop. Presto, a fresh valve. Be prepared for quite a bit of water dropping out from above.
Then take SiHa's advice and don't leave it hard open.