There is… in roughly ascending order of severity / hassle:
EDIT: oh yeah… also try just rebooting your system. 
-
just force-exit and then retry building (obviously)
-
force exit and then try to reimport the atlas, then retry build
-
force exit and then try a reimport-all first, then retry build
-
force exit, switch to another platform, then switch back and retry
-
delete the atlas asset and make a fresh one, dragging all the same sprites back into it.
-
beyond that perhaps try to break your assets into two atlases?
It gets more and more noodly… ideally if you can remove half the contents of the assets and it still breaks, restore them and try remove the other half instead.
This all presumes you use proper source control or you have absolutely rock-solid backups in place. If you don’t, read on:
PROPERLY CONFIGURING AND USING ENTERPRISE SOURCE CONTROL
I’m sorry you’ve had this issue. Please consider using proper industrial-grade enterprise-qualified 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.).
You can also push git repositories to other drives: thumb drives, USB drives, network drives, etc., effectively putting a complete copy of the repository there.
As far as configuring Unity to play nice with git, keep this in mind:
https://discussions.unity.com/t/736093/3
I usually make a separate repository for each game, but I have some repositories with a bunch of smaller test games.
Here is how I use git in one of my games, Jetpack Kurt:
https://discussions.unity.com/t/807568/3
Using fine-grained source control as you work to refine your engineering:
https://discussions.unity.com/t/826718/2
Share/Sharing source code between projects:
https://discussions.unity.com/t/719810/2
Setting up an appropriate .gitignore file for Unity3D:
https://discussions.unity.com/t/834885/5
Generally the ONLY folders you should ever source control are:
Assets/
ProjectSettings/
Packages/
NEVER source control Library/ or Temp/ or Logs/
NEVER source control anything from Visual Studio (.vs, .csproj, none of that noise)
Setting git up with Unity (includes above .gitignore concepts):
https://thoughtbot.com/blog/how-to-git-with-unity
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. Digital storage is so unbelievably cheap today that you can buy gigabytes of flash drive storage for about the price of a cup of coffee. It’s simply ridiculous not to back up.
If you plan on joining the software industry, you will be required and expected to know how to use source control.
“Use source control or you will be really sad sooner or later.” - StarManta on the Unity3D forum boards