There are two main ways to (re)activate AMP:
Via the web interface
In ADS (the main management interface) - go to Configuration → New Instance Defaults and select “Re-run setup” - this will let you reactivate AMP in-place.
On the Command Line / Shell
As the amp user - run ampinstmgr reactivateall. You will be prompted for your licence key, and all of your instances will be (re)activated using the supplied licence.