You don't say which sim you use but, assuming you use FSX, where this problem usually happens more often, the cause is a tweak in the FSX.CFG which is not right for your system.
- Reset your video driver to default
- If that doesn't work, try to delete your FSX.CFG, so a new one will be created with all default settings. Apply only the HIGHMEMFIX = 1 tweak and nothing else, if you use FSX. After you reset the FSX.CFG to default you must reinstall GSX
- If that doesn't work, be sure you are not running the sim or any of the addons that starts with it in Compatibility mode.