What sort of pan is it ? If it's a stainless steel one, then try - very gently - rubbing at the stain with a damp brillo pad. If you don't want to be so aggressive, then add a couple of tablespoons of ordinary soap powder (as used to wash your clothes) into the pan, half fill with hot water, bring to the boil and simmer for five minutes. Turn off the heat and allow to cool - hopefully all the blackness should have lifted off. I've heard also that adding vinegar or coca cola to the pan and bringing that to the boil for 5 minutes will also work.