Missing photo files

I have an iMac OS X 10.9.5...2 GB of memory

Only 1/2 of the storage is being used.

Going into my iPhoto the other day and I saw that at least 2/3 of my photo files were blank...no photos.

They are also missing from the general library, though an occasional photo from the missing files was noticed.

How can I get them back? What is going on?

Thanks for any info.