My Magento site broke right after the upgrade. What do I do first?

Put the store into maintenance mode and stop running commands, repeated setup:upgrade attempts can deepen a half-applied schema. The most common cause of a break right after a 2.4.x upgrade is a stale generated/ directory plus an un-flushed cache: the autoloader is still pointing at old interceptors. The safe sequence is to clear generated/, re-run setup:di:compile, then cache:flush. If you would rather not touch a live store, send us the source and target versions and we reproduce it on a clone first.