Someone used your service to file a fraudulent 2018 return in my name . As a result my 1st stimulus was sent to Green Dot . Now what ?

Unfortunately, if you yourself did not file a tax return via TurboTax in 2018, the first step you must take is contact the IRS.  You will need to first establish the tax ID theft, and work through that process.  Since the first stimulus was sent either via the 2018 or 2019 tax return information and is issued by the IRS.  We don't represent Green Dot in this forum, but they really aren't the ones who issue the payment; the IRS is.  Thus, they can figure out how to get it re-issued to you. So, hopefully, you can contact them.  This webpage provides additional information on how to do so for this type of issue:  Identity Theft Central - IRS