I had the same. App in LHR. Was down at the same time a few hours ago.
I checked flyctl --app name status and the two instances are showing as created 3h44m ago. I haven’t touched the app today so those must have been re-deployed by fly as the prior ones were much older.
The status page shows no incidents reported today but perhaps you are still investigating or don’t want to publicise it yet.
I have two instances in the one region as per advice: Again issue Deploying to MAA Region - #10 by kurt Though that hasn’t helped here 
I thought I had backup regions set. And that seems to still be the case:
$ flyctl --app name regions list
Region Pool:
lhr
Backup Region:
cdg
The app has no volumes.
I guess that means the backup region did not kick in? Is there any way of seeing in the logs if it did (I’m assuming it did not based on the app being down)?
I checked flyctl --app name releases and that only shows mine from a week ago. Likewise flyctl --app name history. Same. I don’t know if those would include deploys/releases to a backup region and so missing data there means it did not happen or would not be listed even if it did. But it certainly looks like not, especially since the regions of my two instances show as lhr now, and I would assume they would be cdg. Which is disappointing.
I’d assumed that Fly would not let new apps deploy to a region if capacity was low. Or at least somewhere within a critical threshold. So that there was sufficient capacity for existing apps in the region to stay up.
Any way to avoid this on the user side going forward?