Best answer as chosen by user ac1818: It's likely hardware
It's likely hardware and it will involve a top case replacement unless you want to amend your story to include the keycap being loose.
Otherwise, there's no way to repair just a single key, only replace a keycap, and the keyboard is an integral part of the top case, so you have to replace one to replace the other.
The good news is, is that as repairs go, this one is probably in the moderate range. Probably around $250US out the door. Keep in mind that's just a ballpark figure, different repair shops will set different labor rates, and may charge you two hours labor on that because it is a lot of work to replace a top case on a unibody MBP. The unibody Macs are essentially built upside down, so the top case is the base component everything else is built around, and to replace a top case means to basically rebuild the entire laptop inside the new case.
Now, if it's just a loose keycap, some places might just charge you a basic diagnostic fee, since it's really a pretty simple matter to do. You just may want to have someone else do it, because there's some delicate bits of plastic under the keycap, and if you go digging around under there yourself, you may damage the rubber plunger and then you will need a new top case, AND it will be considered abused, so will cost you probably 50-75% more for the part. If some repair shop damages it, then they are on the hook to replace the top case on their dime.