Problem uploading to board

Hi everyone,

I have a new arduino uno board, and it was working fine, until all of a sudden an error would appear every time I try to upload something:

avrdude: ser_open(): can't open device "\.\COM9": The system cannot find the file specified.

I have checked multiple times that the board is connected to COM9, and have tried uploading after disabling the device, re-enabling the device, switching ports, as well as reinstalling the driver. I guess the port must be in use by something else, but I have no idea what. I've spent 5 days straight trying to fix the problem so any help would be greatly appreciated!