That is too general of a question.
What kind of lock is it and how does it lock and unlock now?
If only power is required to unlock the door, you could just use a smart plug to turn on the power to the lock. Then you can also tell the smart plug to turn off the power so it will lock the door.
But be careful of how you want to do this. If other people can hear what you say to Alexa to unlock the door, they can do it as well. Maybe even from outside the door if they shout.