My heating unit is not working,took cover off unit and saw black wire coming from breaker box was burnt ,what should I do?

If the wire is burnt there is a reason. If you do not know about electric and have the proper test equipment it is time to call a local pro for a on site inspection and repair.

Perhaps the wire in question was never securely connected, and resistance led to heat build-up. Renewing the connection may solve the issue.