Try "Safe mode with networking" - can you activate from here?
Can you get an option to Repair the existing installation from a recovery partition? If so you will get a message about activation as go go through the repair process - accept it - and assuming the laptop can connect to the internet it will re activate. If you dont activate at this point you will have the same problem as you have now.
If you have a disc - it must be the same type - home or pro - you could run repair from that and use the coa on the laptop.