At 6am this morning the car battery was dead. (It snowed all night and temperature was 3 degrees this morning.) 3 hours later I went to check it out and it started right up so I let it run for 20 minutes. Now 3 hours later the battery is dead again. Everything is shut off! So is my battery bad or is it something else?
First I'd clean your battery terminals, since even a cold battery at fullish charge will try to start. After cleaning, make sure they're clamped tight and there's no corrosion.
Also, describe "won't start" - clicks, nothing, slow turnover, turns and stops, etc.