I upgraded the firmware and now it won't turn back on properly / is stuck in a "boot loop"

MagWLED-1 uses an ESP32-C3 module and WLED releases aren't tested as rigorously with these modules yet. To stop the boot loop, press and hold the boot button on the board as you plug it into the USB-C port. This will put the device into boot loader mode. Using the WLED Installer Site, choose a known good ESP32-C3 version, such as 14.0.