I need help installing my pmdg 747 for p3d. Before it works perfectly but after a month I had to reinstall it again and now it doesn't work anymore. When i chose the aircraft in p3d it gave me an aircraft with black gauge and there is nothing to enter my product key.

You need to provide more information.

1) Did you re-install onto a new PC?

2) Did you disable your license before uninstalling?

Your antivirus can corrupt the install, So repeat the install with it disabled. Otherwise you can raise a ticket with PMDG and they can supply you with a reactivation tool.