Update: Just got off the phone with a support engineer - this is a known system-side bug impacting a subset of advertisers. Starting this week, and occurring "randomly and intermittently", some accounts had their Ad Distribution setting set to "Not set", basically pausing all ad groups and campaigns in the account. This can be seen in the Medium column on the Ads tab. Unfortunately manually setting ad distribution to Search Network or other does not fix the problem. No known estimate to fix this...sigh...