How do I free up a "frozen" bathroom sink water shutoff valve , so that I can repair a leaky faucet ?
Thank you in advance !

You could try WD 40 or PB blaster

You'll have to muscle it open with a wrench, might be a good idea to turn the water off and have a replacement valve on hand in case it breaks.

Pliers should do. Be careful so you don't break off the handle. If it doesn't go than you will have to shut off water for the whole house. Then, repair your faucet and replace the frozen angle stop with new one.

You can buy a special wrench that fits over the stop handle. This wrench basically increases the diameter of what you hold in your hand so it increases your torque. The Gordon Wrench - Home You can buy these over the net or at ACE and Menards. The other thing you could try would be to close your main and unscrew the cap nut just below the handle. This nut will come all the out and the valve stem will be with it. You could soak overnight in vinegar or CLR to loosen. I would not worry too much about breaking that flimsy handle, they are replaceable.