Hi.
Com5 implies Windows OS.
Press and hold the windows key, and press pause|break (Pause|Untbr) .
Now click Device Manager (i don't know the German term for that).
Click Ports (Schnittstellen i suppose), and look for your Arduino.
It will have the Com-port number shown next to that entry in the list.
If you still can't find it, unplug your Arduino and reconnect it while keeping an eye on this list.
The port which disappears and reappears, will be your Arduino.
If it still isn't there either the drivers have been corrupted, or the USB to serial solution on your Arduino board has a problem.
Depending on which USB to serial solution is on your board (i don't know which ones are available for the Due), this might be a software problem requiring reprogramming.