I guess it depends on what you think an unhealthy number is! Keep in mind that number varies based on subject, test length, which test it is in the semester, who you teach, etc.

Don't focus on feeling like a failure. And I have had tests, teaching days, etc. where I felt that way. It happens.

Take a minute to look at what might have happened. Knowledge is power! Figuring it out helps me feel more confident I know what is going on and helps me help the students better.

Did students consistently all miss the same questions or questions on the same topic? If so, are the questions unclear? Are the answers unclear (for example, the answer is A, but everyone is saying D)? If this is a huge problem, you may want to consider throwing a question out, giving credit for more than one answer, or curving the test.

If they're missing the same questions or questions on the same topic, you probably want to go over that topic in class - lecture on it, let them ask questions, and have them do another assignment related to those questions. Try teaching that material in a different way. Lectures didn't make it sink in? Try a group discussion. Homework assignments didn't work? Go over it in class.

Were a lot of students commenting about not studying? Did the test come at a point when they had a lot of tests or papers due in other classes? They may have, honestly, blown off your test. I've taught a lot of gen ed classes and find that it tends to be a bit lower on some students' priority lists than, say, classes in their major.

Seriously, walk away for a minute. Go do something relaxing or that you're good at. It will make you feel better about you and about your skills and you'll be able to come back to teaching in a better frame of mind.