A Microsoft file hosting and synchronization service.
A quick update on this. Finally, over five weeks since I first saw problems, my storage appears to be back to normal. It took a total of 3 days to re-synch all of the content, and caused me problems while it was going on, but I am back to where I would expect to be.
Possibly the best message I can offer to people is to "hang in there" and all will come good. That said, Microsoft should be utterly ashamed at the poor approach to managing this situation and the dire communications with users, particularly once they had reported it to support.
So much time would have been saved, both by Microsoft and their customers, if they had just provided an honest and accurate statement that explained what was happening and how it would actually be resolved. Taking the approach they did is disingenuous, and as a consequence they have lost a great deal of trust and confidence in their ability to deliver a reliable service.
Shame on them - I expect many customers will have migrated to other platforms because of this debacle, and that serves them right. Of course, we're only little people who don't really matter to them - their only saving grace in that respect is that every other major IT provider is also as (or more) arrogant and self-serving.