Serious issue. If you had good vision in one eye which is now fading or worse if it suddenly went away, you should see an ophthalmologist for evaluation. The list of possibilities includes things like strokes in the eye, diabetes, optic neuritis, bleeding and infection. Most conditions with loss of one eye are serious and need attention.
See eye MD. If you have loss of vision in one eye, you need to see an eye md right away, especially if there is no pain. There are many conditions such as a retinal detachment that have a limited time window for treatment for optimal outcome. If your vision is just blurred, then it is not nessarily as urgent. Either way, you need to be evaluated.