If you were on good terms before you leave then it isn't an issue. Just lay it out in an email or phone call if you get their number. "Hey Prof. Whoever, I know this is kind of out of the blue but I am applying to medical school and was wondering if you could write me a strong letter of recommendation. I will send a resume, my personal statement, and whatever information you would like. It would be greatly appreciated."
The worse thing that happens is that they say no.