My Preview's Tools stopped working,How do I fix it?

Start with this...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

Are you running any third-party utilities like anti-virus, so-called "cleaning" apps, or client-side VPNs (the ones that spend $millions$ on TV adverts)?

Is this with the iMac running macOS 10.11 "El Capitan" shown in your equipment line, of some other Apple device?

OK.:)

¯\_(ツ)_/¯