Couple of things to try:
Restart your mac and try launch iPhoto again.
If that fails:
Go to your Pictures Folder and find the iPhoto Library there. Right (or Control-) Click on the icon and select 'Show Package Contents'. A finder window will open with the Library exposed.
Look there for a file called ‘iPhoto’Lock.data or similar. Drag it to the Desktop. Make no other changes. Start iPhoto. Does that help?
If not: Where is your iPhoto Library stored? If it's on an External Disk, what format is that?
If you have Aperture 3.3 or later and iPhoto 9.3 or later you can merge libraries with Aperture.
Otherwise the only way to merge Libraries is with the paid ($30) version of iPhoto Library Manager