If you have Paratext installed, you can view your existing code on the menu under Tools -> Registration Information. But if your computer dies or is stolen, you should generate a new code instead of reusing the old one. This will invalidate your old code so that it can no longer send and receive data. The new code will be linked to the same user and projects, so nothing needs to change at the project level. Just enter your new code in the Registration Information dialog, and you can resume working.
To generate a new code, log in to registry.paratext.org and click on your name in the top right corner. Then select Registration. You will see a button that says Generate. Click it and you will be able to copy the new code immediately. It will also be sent to you by email.