Hi on my 80D I was able to see 3 different grid patterns in the viewfinder. 3x3, 6x4 and 3x3+ diagonal. After making some adjust to other settings I can no longer see these. Canon helpdesk tells me these grids are only available in Liveview. I disagree as they were definitely in the viewfinder as well. How do I make them visible again? And yes, in the Viewfinder Display option I do have it set to Show.
What other adjustments and to what feature did you change, this info might help us steer us in the right direction to help you.
I would suggest doing a reset to factory settings. I'm not sure where that would be on an 80D but it will be there somewhere.
Look up the viewfinder in the user manual... There's a number of things you can do like turn on an electronic level. As to turning on your grid...
If you have Face + Tracking selected I believe the 80d will disable grid in the view finder. Try selecting another mode.