Sudden vision loss in one eye. Who do I call?

ER. I highly recommend you go to an emergency room for evaluation.
Emergency. Sudden vision loss is dangerous and emergent. You should go to an er immediately but if it is day time and you have an ophthalmolgist you usually go to, call and see if he is in and immediately park yourself in his office.
GO TO ER. If you have an ophthalmologist that you can call then call him or her. Otherwise you should not waste time. You must proceed to the nearest er.