GPU Acceleration (Mac) Suddenly Disabled - Rendering Times Killing Me

First, have you checked the renderer setting in the general tab in the project settings?   Have you tried creating a new project and see if they correct option returns?  

Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card

Have you done any recent OS or Premiere updates?  If so, reverting to the previous OS or Premiere version may be necessary

I strongly recommend that when you have a stable system, that you create a bootable clone of your startup drive using something like carbon copy cloner

https://bombich.com/

and that you update it on a regular basis before you do any updates or install major software.  This way, if anything goes south or gets corrupted you can get back to work relatively quickly.