Hiiii. Hope someone can help. Noob here…
So I’m following this tutorial on a tiger and we’ve created the fluffy side parts/cheeks with a few mirrored cones.
Now somehow they became separate now and
When I tried to use some tools on it, like “move” tool, I noticed that the changes are made only to one of the mirrored cones.
You can see the affected part in the red circle on the photo.
Help please 
I believe once validated, mirror objects are separated and considered individual objects so the symmetry between them is no longer there. To get it back, select both objects and choose Join from the object menu. Then make sure symmetry is on.
EDIT: I was wrong. They still should be mirrored after validating unless you Un-Instance them. Did you hit the Un-Instance button on accident? If so, you can still join them and turn on symmetry.
This should be no problem.
It seems you validated the mirror node.
In scene menu, you see little arrows and lines on left showing you which object is connected to which node.
Your parts are connected to a group node now.
Before doing anything, very first step is:
Use “save as” and save with a different number or comment.
I would do this twice.
Tiger01 would be the scene like it is now as a backup. You can always go back. Tiger02 is the version you are working on from now on. Just in case.
I would suggest two options.
With one cone selected, press plus sign and add a new mirror node.
After, you can delete, the cone not connected to mirror node.
Tap on one cone, hold, now move it up to another mirror (red) node, like that one for eyes. While hovering in scene menu, see left lines connecting and disconnecting. When under mirror node hoover as long as left line of cone is connected to mirror node, release. This cone is mirrored again.
Now go back to blue group node with left cone and delete both.
If successful , save! 
I somehow fixed it!
So in the end I ticked both cones in the scene menu and then pressed the ‘+’ sign to add a mirror.
After that I had to delete one of the ticked cones and it worked!
Thank you Roger so much for your reply but it didn’t work for me. I found the solution tho 
Thank you sir so much for your reply but it didn’t work for me. I found the solution tho
That was almost my solution 1 just vice versa. So I planted the idea into your subconscious 
No matter what, good you are back in business.
Defo the credit goes to you, my friend 
