Short answer: a bug in the latest version of Micropython for Raspberry Pi pico. Recover by installing an older version.
Update. The latest version of Micropython does not cause this problem. Seems to be limited to this single version.
I bought a batch of Raspberry pi picos in Nov 2021. Been working with 2 of them ever since with no problem. Decided to update the micropython firmware. The units then lost connection to the computer with no COM port available. They will now mount in boot mode and Thonny will download the file to the pico but then fails with a message that a com port cannot be found. Tested with both Windows 11 Thonny, Raspberry Pi Thonny and manual drag/drop. Neither computer can find a pico com port. Tried 2 other unused picos in the same batch and had the same problem from the start. Tried using zadig to install a new USB driver on the units; no success but a new error message: “Device Descriptor Request Failed”
Decided it was probably a bug in the 1.24.0 version of micropython. Found the compiled archives of past versions and installed 1.23.0 by drag and drop. This version installed correctly and had communication with Thonny thru the USB connection. When doing an internet search for compiled past versions, Google will take you to github mostly where you can compile the version you want. The archive you actually want is on the micropython site here.
Jerry's blog 