HI there,
and welcome here.
If while in bootloader mode you can see it show up as a drive letter in Windows, Head over to the drag and drop thread and unzip the uf2 file for the RGB led blink test and drag that to the drive it will load, reset and start blinking the RGB Led in sequence and print the color out of the serial port.
After that, uploading and programming can begin Normal again. The problem was something messed up the structure of the flash or memory. You can restore it also with the Jlink , however the Uf2 is way easier.
Which BSP file was installed when you added the Micro/Circuit P stuff. Look at the WiKi and try the conversion again.
HTH
GL 
PJ
