In order to get LabVIEW working again, you will need to contact the third party to obtain the serial number for the software. Unfortunately, there is no way to get a fresh serial number without repurchasing a new license.
While you are waiting for your serial number, you may install an evaluation copy of LabVIEW which has almost all of the functionality of the full version with only some evaluation limits
. By default, the trial period will be seven days, but by signing in with an NI profile, the evaluation can be extended for another 45 days. If you are unable to extend the evaluation copy or are having other activation issues, please contact NI Support.