Step 1 - Verify Integrity of Game Cache
Go to Steam client > Library section
Right-click on the game and select Properties from the menu.
Select the Local files tab and click the Verify Integrity of Game Cache... button.
Steam will verify the game's files - this process may take several minutes.
Once the process is completed, the Check Window will automatically exit.
Update: If you get a crash dump when trying to run FM12 after this, restart your computer and repeat Step 1.
Step 2 - Rename ClientRegistry.blob
Completely exit from the Steam client.
Go to C:\Program Files\Steam (or the Steam directory that was specified during installation).
Locate ClientRegistry.blob and rename this file to ClientRegistryOld.blob.
Restart Steam to allow the file to be recreated.
Step 3 - Refer to Steam's Knowledgebase
Please refer to this article from Steam’s Knowledgebase and follow thoroughly all the steps mentioned.
Step 4 - Remove local content
Failing that we suggest attempting an old-fashioned uninstall and reinstall (You can delete the game by right-clicking on it in Steam and clicking 'remove local content' - this will not remove any of your old save games. You can then restart the installation process by again right-clicking on the game within your Steam Library).