Rhinoceros 6 said Rhinoceros License Not Found

first i downloaded Rhinoceros 6 and i got email for license and can use this program about 3 days but suddenly its close and when i open it again it said
Rhinoceros License Not Found
Rhino requires a license to run.
Rhino will now exist.
i try to log in account and re-downloaded again and again but still be like that. what should i fixed it?

Thank you.

Hi - for licensing issues, please contact sales@mcneel.com directly.