Tight shut off valve

My toilet has been running for a few weeks and is driving me crazy! I've tried to shut off the water by turning the shut off valve but can't seem to turn it either way. Is there a way to loosen it? I've thought about hitting it with a hammer but I'm thinking there's got to be a better way. Any suggestions?

Use a pair of pliers. Either it will turn and shut off, or the handle will break off and you will have to replace it. But, since it is useless the way it is, and you might have to replace it anyway, you might as well try.

Add some liquid wrench the night before. And note where the house shut of is at in case you get the whole valve in your pliers.

I'm no where near the expert status anyone around here is ... I'm just an anal-retentive-worry-wart-weekend-warrior ... but ... I'd shut the house supply off, drain lines down a little lower than wherever this valve is and replace the thing while working on the leaking toilet. Again this is an overly cautious opinion backed up by no expertise but its what I would do here just thinking that its so simple to prevent so many other potential problems by replacing a valve which is that stuck or that tight. Hope this helps.

And replace the valve with one of these 1/4 turn ball valves.