The J-Link EDU mini can be had for < $20 for non-commercial use, and can be used to re-flash the boot-loader if necessary; you’ll need to either solder wires to the SWD pads on back of the XAIO, or construct some sort of a jig; but I’d try rebooting your host computer first, as it appears sometimes the host’s USB Driver Deamon gets confused, and won’t properly recognize the XIAO’s serial port from time to time (at least on my Mac).