Leonardo suddenly won't upload

[sterretje]

If it worked before with a USB3 port, it should still work with a USB3 port. If you have access to a USB2 port, you can give that a shot.

@IanMCox117

Forcing reset using 1200bps open/close on port COM3
PORTS {COM1, COM3, } / {COM1, COM3, } => {}
PORTS {COM1, COM3, } / {COM1, COM3, } => {}
...
...

On the Leonardo, you can press and release the reset button when the line "Forcing reset using 1200bps open/close on port COM3" shows. You have all the time in the world while it is showing those PORTS lines.

To play it safe, you can select COM1 in the IDE when uploading using the manual reset.

You can check the Leonardo's behaviour in Windows device manager. It should change from Arduino Leonardo (COMx) to Arduino Leonardo Bootloader (COMy) and after a few seconds fall back to Arduino Leonardo (COMx).

@IanMCox117

Exception type - Port busy.

That is the relevant error for the attempt to upload again after a failed upload.

"Port is busy" might be solved by disconnecting and connecting again; else the only way I know is to reboot the PC.