Apache was down for most of the day, as I didn't get home from work until just now. It appears the monthly log rotation didn't properly restart things after it finished this morning. I've fixed it and things are now back up and running. Sorry for the inconvenience, and thank you to all of you that sent me messages to let me know.
EDIT: I've set up server monitoring on my phone now. Hopefully that'll help me catch things a lot quicker next time there's a problem.