That's either a problem with that specific project or there's something wrong with your computer. Try opening an older project or creating a new one and working with that to see if the problem still occurs.

If not, then you may want to go back to an earlier version of that project from the auto-save folder and see if you can recover something from before the problem started. See this post for more details.

If the problem is still present with a different project, try running the commands shown in this post.