HI there,
Very Good. Yes , there are some threads on here so check those out, You can download the Adafruit(Seeed Ships with) Bootloader and flash it Via , SWD to the Xiao and get the (double reset-fast) bootloader mode back. Send it a BLINK test or better yet when you get to bootloader mode successfully , drag & Drop the RGB_TEST.Uf2 file (from the test thread)RGB_Blink_test
after it completes It will reset and start Blinking RGB colors on the user LED.
HTH
GL 
PJ 