Well, never mind everyone. It's fixed. I managed to get someone to help me jump start the car. The battery was running 10v when the car was off and 14v when the car was on. Interestingly, the charger I was using said that it had 75% - 100% charge left

Anyway, I bought a new battery, and all seems to be well. Now I'm just hoping that my alternator is fully functional. I could have been running on just my battery for the last day or two (after reinstalling the alternator), which could explain the sudden failure of the battery.

Or the battery could just be old. Here's hoping!