Well, it's time to find and check the main shutoff for the house. Hopefully, that one works. ONce you have that off, replace the shutoffs. 1/4-turn ball valves tend to be less likely to corrode, but there's no guarantees. Once you do get the main off, you could try some pliers on those under the sink. Maybe a shot of pbblaser (a spray penetrating oil) may free it up. Works best if you can leave it overnight to penetrate. Don't try to force those until you have the main off, as they could break - if it does happen, it's usually just the shaft and won't leak, but you don't want the worst case scenario. Those are threaded on, so pick up some new ones, some pipe dope (think it will work better than tape on those old threads, but you could use both), and spin them off. Only gotcha is if it spins out of the wall...use a second set of pliers to hold the pipe when trying to remove the valve. It could get messy if that nipple comes out of the wall. If it is galvanized, it may actually be a good idea to replace it with a brass one. LIttle things sometimes become bigger ones.