Just in case if anyone else might have run into the same problem, the following article from the SDL Knowledge Base has helped me:
Click Start > Run...
The Services dialog will open.
Scroll in the list to:
Nalpeiron Licensing Service
Nalpeiron Control Service
Right-click on Nalpeiron Control Service and select Properties.
In the Properties dialog box, switch the Start Up Type option to Automatic.
Click Apply and OK to confirm the changes.
Right-click on the service name again and choose Start.
Repeat step 5 to 8 with the service Nalpeiron Licesing Service.
Close the Service dialog and start Studio again.
Please note that in my case, the error code wasServiceVersionNotCurrent -65
If you get a different error code upon trying to deactivate your license, you may need to look for a different solution.
Thanks very much Alya for this solution, Studio is now working for me. One thing I am wondering about is that Guillaume said above that he had returned my activation manually, yet my version of Studio is still activated.