Same thing happened to me few times in the past as well. Thank you that I now know that I'm not the only bad one here. Bad for you of course.
In my case the map files were still there, but all attempts in moving them to other folder and re-setting the maps location in Osmand was to no avail. At the end only re-installing Osmand worked.
Since on my trips I'm dependent on knowing my position I never rely on Osmand only or on one device with Osmand only. Usually my good old Garmin unit is saving me.
V.