You can use your key on any number of computers but not simultaneously. If you run Hand2Note with the same key on two PCs at the same time, the program will shut down on all devices.
If you forgot to exit Hand2Note on one of your computers, then you can reset the key remotely through your account on our website.
You need to:
Close Hand2Note
login to your account(
https://hand2note.com/Profile)
Click on "Reset serial login data"
Launch Hand2Note
That will fix that issue.