I have 2 Arduino cards connected to USB, it is COM5 and COM6. 2 days ago they both can be seen by Arduino IDE and were listed in device manager. I can communicate with both of them. Today, only one can be seen by Arduino IDE but the device manager still can see COM5 and COM6 are used by Arduino cards. Both cards' lights are on.
But only one card can communicate with COM port.
What is the problem?