Unable to Add new photos

IPhoto 5 is not loading any new pictures, neither from my camera or off the HD. It says they're loading and they appear in the Library, but it stalls (hours). When I try to close the program it says, Please wait until loading complete. I have to force quit. Then when I re-open it, all the pictures are gone from the library, but they are on the HD. This is a recent occurrence; adding new photos used to take seconds. Why? Please help.