Can't record on GarageBand

I have a MacAir, MojaveOS, 10.14.1, about 3-4 years old.

I don't use GB that often, and mainly only for basic recording. However, having said that, I've probably recorded on it and other recording systems many times over the years. If not in the hundreds, at least dozens and dozens of times, including the one I'm currently having the problem with. And they basically all work the same, in my experience. I mean, you open a new track, hit record, and that's usually it, right? And I recorded several tracks just a few weeks ago on this machine. But a few nights ago I tried to record something and for some reason it just doesn't work.

My audio input seems to be working fine, as I can still record and send myself an audio text message. And my voice registers in the audio section of System Preferences. Also, my basic audio output must be working, since I have full audio playback on my machine overall. In fact, I can even listen to other songs I've recorded in the past on GB, track-for-track, and they're all fine. The ONLY thing I cannot do is record. When I click the red record button, the track starts to move, like usual, but it's just a flatline--there are none of those sound-wave graphics. And when I play it back, just silence.

If anyone can help, please let me know. Thank you!!