Hi MaggieLyn21,
Welcome to the Support Communities!
Are you working on your internal Macintosh HD or are you using an external drive for your iMovie projects?
Do you have your data backed up via Time Machine or some other means?
You can use the Finder application to "Get Info" on the internal or external drive to check the capacity, and amount of free space available.
You should also review the "Sharing & Permissions" settings for the drive.
Disk Utility can be used to check either drive for permissions issues.
The following artilces may help you with this:
Mac OS X: About Show Info, Get Info, and the Inspector
http://support.apple.com/kb/TA20359
1. Click the Finder icon in the Dock.
2. Select an item in the Finder (ex. Macintosh HD or your external drive)
3. Choose Get Info from the File menu (or press Command I)
Troubleshooting permissions issues in Mac OS X
http://support.apple.com/kb/HT2963
Apple - Support - Mac Apps - iMovie
http://www.apple.com/support/mac-apps/imovie/
I hope this information helps ....
Have a great day!
- Judy