Note that there were some bugs in earlier versions of Audacity that could exhibit this behavior. Make sure update to the most recent version: 3.1.3. Audacity ® | Downloads 
If you still have the problem with 3.1.3 you can try assuming that the project is corrupt and exercise the instructions here: Corrupt or Otherwise Broken Audacity Project Recovery but it is more likely that 3.1.3 will bail you out of trouble.
If this is Audacity 2.x or prior with an .aup project and _data files, these steps are not applicable and you’ll have to wait for someone else to post.