Yesterday, Microsoft released build 19002 for Fast Ring Insiders, but late Friday afternoon they've updated that build with a fix for an "issue causing some devices to get stuck during shutdown or restart." The problem build only contained one "new" feature, a new Bluetooth pairing experience that first surfaced with build 18985, but is now rolling out to 100% of Insiders. 19002 did contain a number of changes, fixes, and improvements, so it was probably one of those that borked the shutdown/restart experience.
Of course, if your shutdown experience is broken, it may be a little hard to install a new build. Insider Team member Eddie Leonard explains how to work around to get the fixed build:
You may have to get the base build of 19002.1 first. Remember to use the cmdline shutdown /r /t 0 /f instead of the restart button and give it several minutes to reboot.
— Eddie Leonard (@DJ_EddieL) October 19, 2019
So if you're stuck unable to restart, open a command line (Start >search for "cmd">click on "Command line"), and type "shutdown /r /t 0 /f" (no quotes) to force a shutdown. It may take up to 10 minutes, but your machine will eventually restart. From there, check Windows Update, and install the build offered (you may have to update twice, once to the "base build" of 19002.1, and then the fixed 19002.1002). Some users are reporting having to reboot twice, as "Update and restart" still shows after an initial reboot, but you should eventually get there.
Hopefully after this everyone, both at Microsoft and Windows Insiders, will be able to relax and have a nice weekend after all this is sorted out. Good luck, and let us know if you continue to have problems.