You're probably referring to Hekate.
I presume you were already using Atmosphere before but now turned off the Switch completely.
If you see a black screen when you turn on your Switch (power + volume up),
your Switch is in autorcm mode and is waiting for the fusee.bin file to be injected over USB.
If you have a PC then you can use TegraRCMUI, on a Mac there is CrystalRCM and on android there is Rekado.
Or maybe you have a standalone payload injector.
If you never saw the green Hekate screen then probably you only have Atmosphere on your Switch.
You can get the fusee.bin file from Github as Atmosphere is open source and legal :
https://github.com/Atmosphere-NX/Atmosphere/releases/tag/1.7.0-prerelease
Biggest problem is that older versions of Atmosphere might not boot with the latest fusee.bin.
Just try the latest and if it doesn't work, just try an older version of fusee.bin.
If you have a mod chip and didn't mess up the SD card, then the Switch should just boot.
If all fails, you need to reinstall Atmosphere on the SD card and the sigpatches in case your a pirate. *harhar*
When completely at a loss, pay a visit to a good repair shop to help you out.