My complete photo library has dissappeared! what happened?

Thanks for posting in the Apple Support Communities machumike,

If you're confident no recent changes we made, start by double-checking to ensure your signed into the account that you expect to be, since each account on a Mac, would have it's own Photos library:

Switch quickly between users on Mac

If you need more help, providing additional information or context can increase the chance that you'll get a helpful response from the community.

For example, do you use iCloud for Photos? Do you keep a Time Machine backup of the data on your Mac?

Cheers!

Is there any way to get my photos from iPhoto?

Do you have iPhoto? And an iPhoto Library?