Today I tried to repair a leaky tub faucet and when I finished and went to turn the main shut off valve back on nothing happened. The handle turns both ways and stops as it should.I don't hear any water flowing when opened or closed. The water was just fine up until the time I turned it off to make the repair. It's a 40-some year old house and it was about 40 degrees out today.And I don't have any water flowing out of any of the faucets. HELP!

 

Sounds like the shut off valve probably broke internally. I would guess that it is the original gate valve. Shut the water off at the meter, and replace the valve with a ball valve. Should fix it.

 

Looks like Lefty's got it. A "gate" valve has part of the stem down in the flow of water when it's open. water flowing around it for all of these years tends to disolve it away (take a look at the Grand Canyon and see what water can do!) and when the valve is shut the "gate" is at the weakest point ( where it's been disolving away) and there is no metal left on the stem "screw" to pull the "gate" back up!
If it's on the utility side of the meter you will have to have the water company come out and turn the water off at the street so the valve can be replaced! Like Lefty says ... replace it with a quality "full bore" ball valve!