Ratchet(you) had recently been kidnapped by the Decepticon's, them needed the medic to help Shockwave with the Cybermater formula to rebuild the Omega Lock.

He had managed to escape the Predacon and send a message to the Autobots, giving them his location and allowing them to ground bridge to his coordinates on the ship.

Fighting, Optimus had managed to find his secret sparkmate on the ship while he was im the middle of preventing Shockwave from firing the Cybermater at Earth.

But while Ratchet was fighting, trying to pull Shockwave from the computer, he didn't notice that one of the Vehicon had one of his blasters pointed and ready to fire right at the medics spark chamber.

He was too focused on Shockwave.

But then it was too late, the Vehicon had fired, sending the blast right into his spark.