Monday morning, the calls started. For some reason, long-time users were unable to edit list items. I figured we had a permissions issue, so I popped in to look at the Site Settings -- and found that I couldn't. A quick trip to Central Administration showed that I was still listed as a Site Collection Administrator, but I had no power at all on the site collection in question.
A quick glance at the logs told me that the server had recently shut down unexpectedly (this is a Hyper-V virtual machine). Apparently, in the confusion, somehow SharePoint decided to lock the site collection as Read Only. This can be remedied in one of two ways:
1) In Central Administration, go to Application Management->SharePoint Site Management->Site collection quotas and locks. Once you have arrived, select the correct application and site collection, and you will have the opportunity to view and set the lock status of the collection (it most likely will be set to "Read-only", and you'll want to move that radio button to "Not locked").
2) Fire up stsadm and issue the following command:
stsadm -o setsitelock -url http://myportalsitecollection -lock none