[Simon yoon]

I made 2 drones by using 2 different CC3Ds, and when i connect each of them to pc, one is connected, but the other one does not connect to PC. It is weird, because it connected this morning.. I used both openpilot and librepilot, but same happens.

Does anyone have any clue about i have done wrong?

[hwh]

Since you have two CC3Ds and one works but the other doesn't we can probably rule out the gcs and USB cable.

Try disconnecting the cc3d from the computer, going to the firmware tab, clicking upgrade and erase, and connecting the cc3d when it tells you to.

If that doesn't work try the same thing but click rescue instead of upgrade and erase.
There's more info on this in the wiki at https://librepilot.atlassian.net/wiki/display/LPDOC/Firmware+Tab 
The page also has what the blinking leds on the cc3d mean.  It tells you to do the rescue first but that step may not be necessary so try the upgrade and erase first.

[Simon yoon]

i tried both ways, but it still doesnt work. the weird thing is, when i open the device manager(on window), the working cc3d is there, but the one with the problem doesnt appear.

so weird, i can still fly drone, but because it does not connect to pc, i cannot change the setting anymore. when i connect to the pc, it seems cc3d is powered (led blinks), but thats all

[hwh]

If you can unplug the working cc3d, plug the other one into the same cable, and it doesn't work that eliminates the pc and cable.

If it's OK in the air then the firmware and cpu are probably good.  It sounds like a hardware problem with the USB.  I'd take a real close look at the usb connector and the components attached to it with a magnifying glass.  There are a couple of tiny resistors in line with the connector, maybe one of them is damaged or not soldered well.