That's a drag. Sorry dude. The first thing I would recommend is going to your my.presonus account and downloading a fresh copy of Studio One Pro (Pro and Artist are listed separately and have different installers, so it's not just the matter of a key change.) And I'd try installing the new download. If that doesn't help I would recommend that you try uninstalling 5 Artist and going into C:\ProgramData\PreSonus\Studio One 5, you might need to enable viewing hidden files and folders (Open Windows Explorer, click on "Organize", and select "Folder and search options". Select the "View" tab, and in the "Advanced settings" window, scroll down to "Show hidden files, folders and drives". Check that option, and hit OK), and deleting the license key and installation info files. And afterwards reinstalling Studio One Pro again from your fresh download. If none of that works I'd just create a ticket with support. We users try to help each other out but sometimes it's not enough.
I recommend, although it's probably not needed, that you backup your songs, files, macros, key switches, and whatever (copying the entire Studio One folder in Documents would be the easiest way) before uninstalling. I've uninstalled several times without anything happening to my files. But if it does, it can be heart breaking. Good luck dude.