It's not just a key, that's just one of its functions. Most keys don't bring life to deceased alien robots.
Here's the thing. The Matrix has always been something of a cure-all. Who can use it and how and to what extent sort of varies from instance to instance over the course of the mythology.
The Allspark cube became what The Matrix of Creation/Leadership had always been in TRANSFORMERS, supposedly due to rights issue when the Matrix movies were still recent. So the Matrix of Leadership's normal role was diminished somewhat. It still retains vast power though, has the connection to the Transformer afterlife, and served as a key to unlock great power.
Wikipedia sums it up pretty well, though. Check it out.
In the comics and shows, The Matrix of Leadership is a MacGuffin of sorts, and has been used for damn near everything, but it generally has to do with creation and lifegiving, as a storage place for immense energy, though it can be used to retrofit things/transform Transformers into more powerful and skilled ones, and has even allowed Transformers or humans to take control of other robots from time to time. It also holds all the knowledge of the Transformers race (A role the Allspark took on). It's been used as a "badge" of sorts, denoting leadership. And it's been used as a key to access supercomputer Vector Sigma, when the original key was unavailable. There's also the idea that it serves as a Gateway to the Transformer afterlife, which is what happens to Sam in ROTF.
There are also other matrixes out there.
So yeah...one of the matrices may be gone. Or it may just be buried under rubble. Tons and tons of rubble.