I have been trying to connect my Pixhawk to Mission planner with no luck, for some reason the pixhawk isn’t being recognized when I plug it into the computer, it doesn’t see that it is there, so it is not giving me a COM port for it. When I go to device manager it shows that I have PX4 FMU COM3 with a hazard sign so it’s not working. I was wondering what I should do to fix this situation?
The same actually happened to me .
I will tell you what I did to solve this (may or may not work in your case).
Based on my understanding as it’s showing a COM port the device is being recognised but not getting connected,
Go to setup (intial setup) and upload new firmware ,wait for it to be installed. Generally speaking while doing this Step device mustn’t be connected to mission planner (the top right corner connect/disconnect button) but because that is your problem in the first place I don’t think that would be a problem.
Once the firmware is installed and pixhawk gets reboot , remove the physical connection from your laptop to the pixhawk and try connecting again, this time instead of selecting the com port , you can leave it in “AUTO”.
If all those don’t work try changing the cable.(but because your FC is being recognised I don’t think you might have to do it, but just in case).
In my case at first I didn’t even get the device recognised in the device manager too, then I tried changing the cable.followed by that it’s being detected but almost the same problem you faced, those above mentioned steps worked fine for me. All the best 
In any case Just let us know what happened !
Edit:-
One more thing worth mentioning is, try to avoid using USB hubs ,at times they cause these issues.
@Aidan28
I assume you are talking about Windows?
This is usually related to missing or incorrectly installed driver.
A work-around like Ludwig suggested can sometimes force a re-installation of required driver.
Note on the side: I use Linux as the handling of USB ports is done differently and usually doesn’t require any drivers. Had a case or two where no Windows computer would connect to a device yet happily connected on Linux.
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