If you are hearing sound, this means that there’s something preventing it from displaying. Here are some things you can try.
Disable Your Antivirus
To check that it's not your antivirus causing this, try simply disabling it and any firewall software you have running. If that is the cause, then you can add an exception as needed!
Unplug Additional USB Devices
There are some devices out there where the drivers conflict with OMD2 loading. We suggest you unplug every USB device on your system and update your mouse and keyboard drivers.
If you've installed OMD2 to a USB hard drive, we suggest moving it to an internal drive as there have been conflicts that prevent it from loading from a USB drive.
If you are using Nvidia Optimus, you will need to disable it. There is a persistent compatibility issue with it.
Disable Steam Overlay
1) Click on Steam and click the "Settings" button. Click the "In-Game Settings" tab.
2) Uncheck the box next to "Enable Steam Community In-Game" and click OK.
(If you would prefer to only disable this for Orcs Must Die! then right-clicking on Orcs Must Die!, click "Properties" and uncheck "Enable Steam Community In-Game" in the "General" tab.)
Turn Off Additional Monitors
Orcs Must Die! doesn’t play well with multiple monitors. Turning off additional displays helps. Even if you have one monitor, it is worth double-checking your settings as occasionally windows will detect additional devices and load them as a display.
Windows 7 directions
1) Navigate to your Screen Resolution settings using the following path: Control Panel\Appearance and Personalization\Display\Screen Resolutions
2 ) Select the drop-down menu under “Multiple Displays” and choose “Show Desktop only on 1”
Reset your resolution settings
Sometimes it's possible to save an invalid resolution in-game. You can modify your config files to fix this using the instructions here.