Could be a few things. One - when you moved the folder with the images and the db files over, did anything change like the path name? Is the HDD named differently?
Start by trying to rebuild the library. Hold shift +option and click the iPhoto icon, or try option +command for iPhoto 5. Rebuild the library - it will take a while - and see if that helps.
In the future, remember, apps like iPhoto and iTunes are relational databases - they need to be properly exported and imported or the links will break.