This usually means your database is not responding (fast enough, or at all), but can be down to all sorts of other things. If you are part way through an upgrade, it may be that a SQL update is taking far too long and the network is interpreting it as a failure.
If it's not that, then have a run through https://confluence.atlassian.com/confkb/confluence-does-not-start-due-to-spring-application-context-has-not-been-set-218278311.html
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