Hi Katie,
I only remember one kind of recent thread with that specific error message, and in that case importing it into a new file resolved the behavior.
Additionally consider these preventative measures to protect your project files:
1) Save and publish projects on your local hard drive. Working on a network drive or external USB drive can cause erratic behavior.
2) Save incrementally. If your app has an AutoRecovery feature, take advantage of it. If not, save a new version of your project every hour or so with a new file name each time. If a file becomes corrupt, you'll still have a working version available.
3) Install Dropbox. Snapshots of changes in your local Dropbox folder are kept for 30 days. If a file is damaged or deleted, you can restore a previous snapshot:
https://www.dropbox.com/help/11/en.
4) Don't leave the app open and unattended for long periods of time. Some users have reported file corruption after leaving their apps open overnight. It's possible that a malware scan or disk backup could run because the machine is idle, making your app vulnerable to crashing.