I thought I would post this in case it turns out to be of use for someone else. I've been building an app that is streadily getting more and more complex and as a result takes a little time to load up when trying to edit it in the studio. However, yesterday, I couldn't edit it at all and was faced with the scary prospect of possibly having to rebuild. However, this morning, I worked out what causes it -
If the app you are editing takes longer to load than the time set in your Autosave settings (File>Account), then the App tries to save before it's finished loading, causing the editor to break. So if the formula bar is greyed out and you can't add/remove screens etc., then this might be why. Simply turn off the Autosave option or amend the settings to give you a big enough time window for the app to load.
(I think there are some underlying data connectivity issues with the sources I'm using which could be causing the slow loading, so at least I can look into those separately now that I can actually work with the editor again!)
