AWS had a multi-hour total S3 outage in us-east-1 in February 2017 that knocked out a huge number of things mostly because it turns out that a huge share of their customers run in only 1 region and it's us-east-1. Things mostly continued to work in other regions.

I recall Azure had some sort of multi-region database failover disaster that took several regions offline, and GCP has had several global elevated latency/error rate events, but I don't think that any cloud provider has been "down" in the sense that the word is usually used.