AppSheet is fully back up again. While we will do a deeper analysis, it appears this happened because of automatic security enforcement rules that (erroneously) shutdown firewall access for our own services. It took a while to discover this, and once we did, we had to manually override this change and disable the automatic enforcer.
This is about all we know at the moment. Exactly why this happened at 2:25PM PST on a Tuesday is unclear and we are determined to find out.
We are aware of and actively investigating a major incident. Will provide status updates here.