There are many threads about broken keys.
Either your bumper has fallen off, or the assembly that holds the bumper has cracked.
You can open a support ticket and/or try to return it.
If you can't afford to ship it back, you can remove the 16 screws on the bottom plate (not the big screws) and check the high C bumper. You will need to pry the spring off the key and slide the key off, there's a little key lock you will have to also pry. Instructions can be found on this forum in an older C Key thread.
If it's fallen off, pop it back on and replace the key,
If the assembly has cracked, you can Superglue it.
I get your frustration. I had two keys break for no reason. The way they broke was as if someone tried to force a key up by pulling hard, cracking the plastic bumper holder. Except that they flopped upwards by themselves.
If these synths pass QC, it's possible they suffered a break in shipping because a key bed material had a weakness. Most people report the key lifting up later, not out of the box. I'd guess there was a problem with a certain batch of the key beds from a supplier, perhaps the automated tightening of the large screws stressed the plastic. They seem to break mainly on the Cs, under which is where the domed screw holes are located.
The Microbrute seems to have less to go wrong, with those simpler Minikeys and far less sliders and knobs to worry about. Return your Mini and get a Micro, and you should have free shipping due to price difference?