Some BIOSes have trouble booting anything other than Windows, so some distros default to clobbering the Windows Boot Manager and trying to manage everything from GRUB. It sounds to me like this is what happened to you except it somehow got borked.
If the Windows install is still present on your drive it should be possible to restore the Windows Boot Manager. Then you won’t be able to boot Linux but you’ll at least be able to get into Windows.
Try booting off Windows install media and running Startup Repair using the following instructions: https://learn.microsoft.com/en-us/troubleshoot/windows-client/performance/windows-boot-issues-troubleshooting#method-1-startup-repair-tool
If that doesn’t work try one of these solutions:
https://superuser.com/questions/460762/how-can-i-repair-the-windows-8-efi-bootloader
https://superuser.com/questions/612830/how-to-reinstall-windows-boot-manager-on-efi-partition
Good luck!
Also relevant xkcd
Between 6-10 hours of what?
edit: oh, you mean the xkcd, LOL. Well let me know if it works out for you haha