@PaulaM, try these steps found on the official Steam support page:
Clear Download Cache
Clearing the Steam download cache will cause the Steam client to flush locally cached configuration data and acquire it from the Steam servers again.
Navigate to your Steam -> Settings -> Downloads -> Clear Download Cache
Repair Library Folder
The Steam library folder contains all of your installed games on disk. These folders need to be writable by all users in order to update properly. Sometimes these user permissions break and need to be refreshed.
Navigate to your Steam -> Settings -> Downloads -> Steam Library Folders -> Right click on folder -> Repair Folder
Verify Local Files
Sometimes files on hard-disks and other storage devices become corrupted because of bad hardware, software crashes, power surges etc. Steam can verify the correctness over installed game files and fix them if necessary.
Find Game in Library -> Manage menu -> Properties -> Local Files -> Verify Integrity of Game Files