Sprite Atlas, Stack over flow

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Try a Reimport-All on your project, see if that fixes it.

If it persists you might need to revert to the previous version of Unity, but that does require that you are using source control (to revert) or that you made a full backup immediately prior to upgrading.

If you are not using source control, please consider doing so now, and when you do, use proper industrial-grade source control in order to guard and protect your hard-earned work.

Personally I use git (completely outside of Unity) because it is free and there are tons of tutorials out there to help you set it up as well as free places to host your repo (BitBucket, Github, Gitlab, etc.).

As far as configuring Unity to play nice with git, keep this in mind:

Here’s how I use git in one of my games, Jetpack Kurt:

Using fine-grained source control as you work to refine your engineering:

Share/Sharing source code between projects:

Setting up an appropriate .gitignore file for Unity3D:

Generally setting Unity up (includes above .gitignore concepts):

It is only simple economics that you must expend as much effort into backing it up as you feel the work is worth in the first place.

“Use source control or you will be really sad sooner or later.” - StarManta on the Unity3D forum boards