Partition Map Failed Because No Slices Were Found
Ugh - the main thing with "your life's work of photos" is to slow down and don't panic. Don't try to fix things when you're desperate or rushed. Don't try something for the first time on that drive. The normal utilities exist to make the drive usable at the expense of losing your files. Or if the files are "lost" not, then at the expense of letting recovery software get some or all of them back.


Recover Deleted Files on Mac OSX [External NTFS]
I may have used Disk Utility to erase my drive - is there hope?
Recovering files from Time Machine backup disk after wrong dd
Accidently formated my external 1TB hard drive!


Instead, reach out to your network of friends to see if anyone has used a program like Data Rescue to repair/recover deleted files. Learn how to connect a drive in read-only mode (you might need to buy hardware or take the drive to someone that does this professionally or has at least gotten three successful recoveries under their belt before you entrust your only copy of your work with them).

You might have lost the photos, but I wouldn't recommend connecting the drive to either a PC or a Mac since both will try to run filesystem checks and if the part of the drive that tells either OS what is on the drive itself is corrupt or just not making sense, that computer could do more damage to your work trying to make the drive "usable" again.


https://en.wikipedia.org/wiki/GUID_Partition_Table


The reason I recommend having a partner is that you will then work together, slow down and be intentional about what steps you take. Just explaining what you are about to do is valuable even if the partner isn't experienced in data recovery.