Have you looked at the faq on Switchresx site, its likely your system is set to a resolution it is incapable of displaying try starting up in safemode see below
"I've defined a new resolution and rebooted, and the system boots with a black screen.First, try to reboot in safe mode. To do that, turn your system off, then restart. When you hear the startup tone, hold down the shift key. Release the shift key when you see the "spinning gear" on the startup screen.The file named Library/Preferences/com.apple.windowserver.plist contains the resolution that the System tries to use at startup. Deleting this file helps generally. If this solves the problem during the reboot, but not in your user account, a similar file named "com.apple.windowserver.XXXXX.plist" exists in your own /Users/yourname/Library/Preferences/ByHost folder. You can remove this file to get back the standard resolution in your user account.If nothing above works, try to press the "reset" or "factory defaults" button in SwitchResX Control for your monitor. This will delete all changes that you have made for this monitor.As a last resort, if you can't even start SwitchResX itself, you'll have to delete the monitor profile that was modified when you created a new custom resolution. This file is located in /System/Library/Displays/Overrides/ and is the last modified file, that you can find if you sort by date. You should remove this file, if you can access the file system, either by1) accessing the file system from an other Mac on the network, from ssh or by mounting a shared folders.2) OR starting up in single user mode (or safe mode)3) OR at last resort: reinstalling the OS above the existing install, which will keep your existing profiles, but remove the system modified files.
SwitchResX. Is a very powerful tool, and some care is required when using it...
