I have an arduino nano with the CH340G. IT has been working fine for the last few months but yesterday it started showing up as an Unknown USB Device under Universal Serial Bus. I have an arduino uno with the same CH340G chip and that works fine, and I am pretty sure the CH340G chip on my nano is still alive because I was getting a stable 3.3v from the 3v3 pin. Any help?
What was the project that you were working on when it no longer worked? If this is a new project, what was the previous project that you were working on.
That is not necessarily an indication that the CH340 is healthy, only an indication that the 3.3V regulator in the chip seems to be working.
Have you tried different cables?
Does Windows device manager give the same VID and PID for both boards?
You can try to update/downgrade the driver; there is the risk that your Uno also no longer will work after that 
I was building an arduino transmitter and receiver using the nrf24L01+ (Power amplification version). I had a separate 5v power source and a separate 3.3v power source for the nrf24, also I accidentally supplied 3.3v to the gnd pin and gnd to the 3v3 pin so maybe is that the reason why it stopped working? The usb was connected when I was doing this, I don't remember any more details after that.
How do I check the VID and PID? Also, is there a way I can check if its a board problem or a usb problem before buying a new cable? Because if it comes to a new cable then I might as well buy a new nano with it
Based on your description in reply #3, my feel is that it will be the board.
In Windows device manager, right click the device, select properties and got to the details tab; check under hardware ids.
Note
I'm not using Windows, so above is from memory
I find these apps invaluable when testing devices connected via usb:
USBDeView View any installed/connected USB device on your system shows all usb devices
USBLogView USBLogView - Records the details of any USB device that is plugged or unplugged into your system shows the changes in usb devices
USBTreeView USB Device Tree Viewer shows all the usb devices - with a LOPT of detail - as a connection tree.
All these are portable apps - dont need installing.
This is taken from a Linux box and shows the VID (idVendor) and PID (idProduct)
New USB device found, idVendor=1a86, idProduct=7523, bcdDevice= 2.64
If you don't get similar numbers, your CH340G is toast. You can compare the numbers that you showed with those of your working Uno.
