Can't see church service or my thumbnail

I'm running Zoom version 5.17.2 (29988) on a Dell laptop running Windows 10 Pro. I have two (possibly related) problems when I'm attending an online service on Zoom:

1. First, I can't see the main camera feed; my screen only shows the PowerPoint slides, not the live action. (I do hear the audio.)
2. The feed from my camera doesn't show up in the main room; I can see myself in the breakout room, but not in the main room.

Thus far, I have uninstalled and reinstalled Zoom several times, with no change in Zoom's behavior. I've checked to see if I might have accidentally pinned a screen, but that doesn't seem to be the case. I can't seem to figure out anything else to try. Does anyone have some suggestions about where I can start checking?

Thanks.