What shape is the body in? If it's pretty well intact then my guess would be that it somehow got stuck there and finally died.
A word a caution from one who knows....try to remember to keep your garage doors closed during hummer season or at least do a ceiling check before closing them for the night. I found another dead hummer in our garage just a few days ago. Apparently it had flown in one day last season to check things out, maybe drawn by the red gas cans sitting inside the door or hunting spider webs, and then became disoriented and couldn't figure out how to get back out. Our garage has one of those center ridgelines that are semi-translucent so that light comes through and I have had wrens and other birds think this much be the way out and ignore an open door. I hated it...that poor little bird surely suffered greatly. That garage is a death trap...hummers, tree frogs, toads. They all wander inside and then die of starvation or, in the amphibian's cases, dessication.
Now I do a quick check before shutting the door.