It depends on the prof, but usually this is inappropriate before the grades come out. It is "inappropriate" in the sense that you will either be turned away or ignored, not that anything seriously bad will happen. Instructors get students emailing them all the time to talk about the exam they are convinced they have just failed before even seeing the results. There are several issues.
The instructor may not have graded anything yet, so he doesn't know whether you passed or failed.
It depends on your professor's policy. My personal policy is that students get the grades they earn. However, if I see a student on the edge I will go over their exam to make sure that no unnecessary marks were taken off, and no mistakes were made. However, I will not bump a student up unless it's something absurdly close, like getting 59.9 when the passing grade is 60.
Your prof may be intending to grade on a curve or some other adjustment anyway. Asking them will not be helpful.
Ultimately, wait until your grade comes out. Until then, there is not much you should be doing. For all you know, you passed the course, then it is a small embarrassment to admit that in your stress you asked for marks.
Now, what happens when the grades come out? As SolarMike said, it depends on your grade and the instructor. If it's a 58 when 60 is a pass, you might be able to convince the instructor to bump you. But yes, if it's much below a few percent or if the test was marked so that subjectivity doesn't count (as in: it's multiple choice or something), then it's unlikely that you will succeed.
I know that it is difficult to wait, especially if you think you failed. But you will demonstrate a far greater professionalism if you respect the timelines of things.