Please make sure you have the most recent versions of DirectX and Microsoft .NET Framework installed on your system.
Microsoft .NET Framework can be found here:
The most recent DirectX can be found here:
If this does not resolve your issue:
- Uninstall Stronghold Kingdoms from your system
- Go to http://www.strongholdkingdoms.com
- Login using your Stronghold Kingdoms login credentials
- Download the Stronghold Kingdoms client and reinstall it