If your cat has had one seizure, you should call and make an appointment. Learn about seizures and epilepsy. This page and the links at the bottom are excellent places to learn about feline epilepsy. Make sure your cat gets quality care. Ask your vet questions to see what tests are being done, what problem they suspect and what future course of action they recommend.
Feline epilepsy is rare, so many vets are unfamiliar with it.
I have received email from people whose vets have not even done blood tests on cats experiencing seizures and from people whose vets have just said to monitor the behavior of a cat that was having seizures more than once a week. In both cases, the seizures were curable, but the vet was doing nothing to help the cat. Bloodwork should always be done, and phenobarbital is generally prescribed when no other problem is evident. If your vet seems unaware of how to diagnose or treat seizures, find another vet who can. Seizures, or the disease causing them, can be fatal if not treated properly.