Hi Petra, I'm so sorry to hear about this cat.
My guess on what happened here would be one of two things.
1. Thrombus to the brain- this is seen rarely after dental cleanings. Basically the problem is an undiagnosed heart disease that has no clinical signs. With these cats, there is some turbulence associated with the valves and it ends up being an area where bacteria can flourish. Dental cleaning takes a lot of bacteria and puts it directly into the blood stream via bleeding gums. The dental associated with the underlying heart disease can cause thrombus (clot) formation and then clinical signs associated with where it goes- in a case like this the brain. Some vets will put animals on antibiotics for a few days before the dental to help prevent something like this- it's efficacy is still not totally proven.
2. If Telazol was used as an induction drug, it can sometimes cause hyperthermia (increased body temperature). This is an idiosyncratic reaction in some cats meaning that there is no known reason why it happens. The high temp can cause brain seizures and thus death in affected cats.
I'm sorry again, I hope this helps ease your mind a little or provides at least some sort of closure.