This sounds like you cannot access the old machine to check the license back in.
If you are a SELECTsubscriber activating against the Bentley Hosted Server or a Local (Deployed) Server, please see below.
One option is to view your Checkedout Licenses via the SELECTserver administration page found at the following URL... selectserver.bentley.com... or for your Local server type "localhost/bss/admin" into a browser window while connected to the server's networl. Once logged in to this page, Find the "License Management" tab and chose the "Checkedout Licenses" option. Here you can view and manually check back in licenses.
If you are a NonSELECT user or cannot log in/do not have administrative rights you will not be able to access/check back in the license remotely.
In this case, please feel free to dial in to Licensing Technical Support (of which I am a member)
We would need to identify your account and would then be able to assist you in releasing the license and activating the new machine.
Please let me know if you need anything else.