So I've been coaching a Cal State Bakersfield dynasty with moderate success, but when I finished thirty-five seasons into it the button to go to the next season is greyed out. Is this because I've been fired or has the game decided that I should retire?

I have no idea. I've never taken an association that far. So, I suggest you ask Gary via a Support Ticket (and submit your files so he can check to see what's going on).