Assuming you have rebooted your system and relaunched FCPX and the library will still not open, or it causes FCPX to crash upon opening, try these things:
1. Restart FCPX holding down the COMMAND and OPTION keys. When presented with the dialogue to trash preferences, select yes. This will start FCPX with the default library. Choose "open library" from the top FILE menu, select "Other" and navigate to the problematic library. If it now opens and works - great, if not, try #2 below.
2. Find your problem library in the finder, control click it and select "Show Package Contents". Locate the event for the project you were having issues with and open it. Inside, locate the "RENDER FILES" folder and drag that to the trash. Empty the trash, reboot your system, and restart FCPX normally. Open the library and see if it now works. If you had FCPX preferences set to Background Render ON, then this may have caused corrupt render files, and by clearing them all manually the library may now open. If so, reset your preferences and be sure to turn Background Render OFF.
If neither of these steps solves your problem, post back with some details of what you did, the project media type, your computer specs, etc. and we'll try something else.