IPhoto was working fine up until a couple of days ago
And what happened in that few days? What changed? Did you install other software? Update software? Run any maintainance programs like cleaner application? Imported new photos or videos?
Or is simply your harddrive running out of space? If you are having significantly less the 10GB free diskspace, free disk space asap - remove large files to an external drive.
To try to narrow down the problem by a few tests: create a new iPhoto library by holding down the ⌥-key when launching iPhoto. Select "Create new" form the panel. Do you also see the spinning wheel for a new, empty library?