restart your computer and reopen the fla.
if there's still a problem
1. test if it's a problem with all fla's including new ones. if so, reset your preferences: https://helpx.adobe.com/animate/using/set-preferences.html
2. if it's only a problem with the fla you were working on, what do you see when trying to reopen that fla?
This did fix my problem. Strange. I don't know how that happened at all.
Uninstall and reinstall 20.5.1.