I can't access any of my pictures. Iphoto has been "preparing library" but it always goes to 49% and then quits unexpectedly. I
Where is the iPhoto Library stored?
Is it in your Pictures folder or on an external drive? If the library is on an external drive, is this drive correctly formatted? The format should be MacOS Extended (Journaled) and the drive directly connected, not a network share, like described here for Aperture (Format external drives to Mac OS Extended before using with Aperture - Apple Support)
If the library is in a supported location, try to repair the iPhoto Library with iPhoto as described here: Re: iPhoto has crashed on a number of occasions, but on 2 occasions also seems to delete several events, including the one I'm editing. When I go to reimport the last event, it shows them as already there. But I can't find them.
Try all four repair/rebild options in turn, starting with the permissions repair: