Good afternoon. One of the buttons on my keyboard broke off. Can it be fixed?

If you are under AppleCare Plus, you may want to find out if Apple would consider it under their limited accident protection. But for the cost and trouble, a simple $20 external keyboard can be had, even with bluetooth. The only time you need wired keyboards are for boot sequences. If the key that fell off doesn't accept direct input without the key cap, and it is one of the boot sequence keys for which there is no duplication you might be forced to get the cap replaced.

Out of warranty you might be able to get an authorized service center to do the work for less. But normally when Apple fixes keyboards I've seen them replace the whole top case of the notebook.

Be especially careful to wash and dry hands before using the keyboard, so anything sticky doesn't short the keys if you use the key without the cap.

There are places online that sell key caps if it is still possible to depress the key without the cap, and it is out of warranty.

I would not attempt to do further work yourself if it is under warranty unless it doesn't impact the built-in keyboard itself.