Its handy to know where your incoming main stopcock is, and the water company one which can be anywhere between the road and the house. My daughter moved in to a 1930's house recently and as part of mapping things out I found her in house one in the understairs cupboard and the water company one under a tile in the small front garden border. A 'yuk' job was fishing out years of wet leaves and slugs from the deepish hole but at least she can now see it. That gives you 2 chances of turning off the main supply. If you have a header tank or two (may not if you have a modern combi boiler) you need to have a plan for a leak on that side of things if there is no valve to isolate the leaky bit: turn off incoming main or tie ballcock up and then run everything that it feeds, flush the toilets etc to empty the tank as fast as you can.
My house isn't as old, but I'm a bit OCD about such things and stuck to the back of the airing cupboard door is a list of taps etc and which valve, if any, isolates it. We have very hard water. Over the years I've also taken out all the 35 years old gate valves (useless by now, didn't fully close) and replaced with 1/4 turn ball valves. Again bit OCD perhaps but if I turn one ball valve off - perhaps to clean out the inlet filter in the shower mixer head - I turn them all off and back on in the hope that the annual manipulation of my balls will stop them seizing up.
Back to the original question, if its on a hot water supply you could run a tap that it supplies and warm the valve up a bit that way. You might try repeatedly tapping (not thumping) the end of the shaft or the body and see if that disturbs any corrosion and try again to turn it, and as has been said make sure you are going the right way - often there is an arrow moulded or stamped in the handle. Then it's back to handle off brute force as above, have a clear route to your stopcock in case.
(Shouting at things sometimes helps - try as hard as you can, step back and shout as many derogatory names as you can muster, curse the person who installed it, curse the company that made it, curse all living beings, then try again.)