Photos lost in Aperture

Hi,

I am working in Aperture 2.1.4 on an iMac Intel Core i5, running OS 10.9.2. I imported thousands of photos from a previous iMac some time ago but if I go to Export a Version, they appear as dotted lines and greyed out unless I click back and forward on them but still won't let me Export them.

The message says, "Versions with unavailable master images cannot be exported.

The selected master image is either offline or not found. Please reconnect it and try again."

I can actually see the image but I can't export it.

Any ideas where the masters might be and how I can fix this as many of these images are important?

I do still have the previous Imac these were exported from so hopefully, I can reimport them so they work - any ideas on how please?

Thanks.