Have you tried verifying the game cache in Steam?
How to:
Start Steam and make sure you are Online.
Select Library at the top.
Right click Stronghold 3
Select Properties and then the Local Files pane.
Click the "Verify integrity of game cache" button.
You might see a message in the taskbar saying that Steam has finished downloading Stronghold 3.
If you like you can "Defragment cache files" afterwards.
If above does not work, try rename "campaign_manager_state.xml" (it keeps track of your progres in the game) and then start the game all over. Play a few maps in a campaign and then try if you are able to replay them.
The file "campaign_manager_state.xml" can be found in Documents\Stronghold 3\Profiles
If none of above works, please tell more exact what you click and what happens when the game will not let you replay the campaigns.
"Cats are the only animals that are both uber, pwns and 1337 at the same time." -King Euric
By reading this fine print your soul is now the exclusive property of HeavenGames.