It is only a natural process of our Mother Nature that animals refuse to eat or drink when they are not feeling well. Usually it takes up to three day to get back to normal if there isn't something serious there. It is strange though that your dog isn't trying to rest or sleep.
You better check it with the vet.
All the best.
I am sorry to hear he is gone, have you found him yet? Hope so, and he is well. If not, it could have been his way of trying to find somewhere to pass, I hope not to upset you anymore, but 12 is old for a dog, and with these conditions he needs to be in the hands of a vet if found.
He's elderly and sick and it could be dehydration, it would affect him this way because it affects the brain as well. However, this could be something else, following the Seizure a month ago if he did not have tests to rule out tumor or clotting in the brain, then it could even be something of this nature and possibly the culprit of the seizure to begin with. Either way, he needs to see the vet, b/c if he won't eat or drink, his health will continue to fail, and maybe rapidly. I hope all turns out well, Good Luck.