This could be a symptom of your turtle having blood poisoning and you will need to take it to the vet as soon as possible to be treated. It is not wise to try and treat yourself.
If a turtles eyes are swollen shut, it could be due to eye sight problems or blindness.A turtle will eat again eventually, and some may recover eye sight with the proper care.
Turtles often swell because they begin to retain moisture when they stay in the water for too long. Let your turtle roam around outside in the sun a little everyday and it should return back to it's little turtle self.