Can you do other things with medical billing and coding other than the medical field?

To some extent billing is billing. If you can process one kind, then processing another is not all that big a change. Medical coding, however, is pretty useless information to have outside the medical field.

One thing that you didn't ask, but I'll answer anyway: there aren't as many jobs in "medical billing and coding" as some places would have you believe. Most doctors and clinics either do their own billing, or have an arrangement with an established firm. It's possible to start a new business doing this, but it's not going to be easy. It's possible you've already found this out and this is why you're asking... you could try contacting one of the firms that you've been hearing "Sorry, Company X does our billing" about and see if they have any openings.