You Rhino 5 license lives here
C:\ProgramData\McNeel\Rhinoceros\5.0\License Manager\Licenses
If you have a lic file there, delete it and restart Rhino. You should be asked for a license key at that point. Any luck?
You may have this solved already but for any license issues it’s best to email sales@mcneel.com for the fastest response. Forum posts sometimes get missed.