Applications with a zero and a slash through it.

I updated to Photos 3.0 a while back.

Ok - that is a different application - Photos for Mac - I will request that your post be moved to the correct forum

What happened just before Photos had a slashed zero?

Most likely you will have to reinstall the OS (which reinstalls Photos since it is part of the OS) - boot into recovery mode and reinstall the OS - About macOS Recovery - Apple Support

LN

It means that it won't run. That could be because it needs updating, or that it is faulty in some way.

If this is iPhoto then what version do you have? And what version of the OS do you have?