I get a 503 or blank page after deploy, what causes it?

A 503 or white screen straight after setup:upgrade / setup:deploy:mode:set production is usually a missing or stale generated/ directory, an uncompiled DI, a failed static-content deploy, or wrong file permissions. We read the real error from var/log / PHP-FPM logs, rebuild generated code, redeploy static content, fix ownership, and bring the store back, then document what failed so the next deploy is clean. See Magento 500 error fixing and Magento white screen fix.