Most likely it will return you to Windows 8 (or maybe 8.1 depending on when you bought it). You'll need to download and install Windows 10 just as you did 3 years ago (unless you did it via Windows update, then it's a different way).
But I'd start with resetting it from the Settings in Windows 10, asssuming you can still access that.
Two recommendations for when it's up and running again:
1. Make a recovery drive. See https://answers.microsoft.com/en-us/windows/forum/windows_10/how-to-create-a-recovery-drive-for-reinstalling/58df9c7d-84de-4652-9952-8bac34abc6c5 or google WINDOWS 10 RECOVERY DRIVE for more information.
2. Make an image backup once in a while to to go back to if necessary. Backup your own data much more regularly.
I hope this isn't a daft query. I have an ACER laptop running Windows 10 which has had a registry / profile malfunction. Despite my best (but limited) efforts I have been unable to fix it and it has been suggested that I do a factory reset. But this is my question - when I bought the ACER it was running Windows 8 but has been updated over the years. Will a factory reset return me to Win 8 and if so will it be easy to restore to Win 10.
I would be most appreciative of any help and advice.