The Pico Arduino core is a recent development, so it may take a long time to compile and upload, or the COM port may suddenly disappear during use.
For the former, we have to wait patiently until a new Arduino core is released.
For the latter case, you can unplug the USB cable, then press and hold the BOOTSEL button to plug it in, unplug it and plug it in again to see the COM port.
If you encounter a problem, you can come first FAQ to find a solution.
The new projects for Arduino are still being updated continuously.