All of the above, plus:
NEVER use the Default selection for audio devices. It exists to show that a fresh installation works, but it's a ticking time bomb for anything else, that does exactly as you describe: it works, and then it doesn't for no apparent reason.
The reason it does that, is because Default defers the choice of device to the operating system, which in turn has its own logic for what to choose. Most of what causes it to change is that you plugged something in, so therefore you must want to use it, or you unplugged something and so the connection needs to go to something else that's still there. When a device that provides an audio function "goes to sleep" or "wakes up", that's indistinguishable from you plugging it in or unplugging it, and so the same logic applies there.
So there are several things that you're probably not paying attention to, that can cause the operating system to change its device selection, and OBS's Default setting dutifully follows that. Then when you go to use it, what used to work is now looking at the wrong device.
NEVER use Default. Go through everything and change that. ALWAYS choose a specific device directly in OBS, so that it doesn't change on you.
And if you don't use something, Disable it or delete it. Just because something was there when you got it, doesn't mean it's worth keeping, and rogue audio sources can really hurt you.
