Is the Cron Daemon Even Running?
If no cron jobs fire at all, the cron daemon itself may be stopped or disabled. Check it with systemctl status cron, start it, and enable it so it survives reboots.
Check the service
systemctl status cron (Debian/Ubuntu) or systemctl status crond (RHEL/Fedora). Look for active (running).
Start and enable it
sudo systemctl enable --now cron starts it now and makes it launch at boot. --now combines start and enable. Use crond on RHEL-family systems.
After a reboot
If jobs worked until a reboot then stopped, the service was started manually but never enabled. systemctl enable fixes that permanently.
Containers and minimal images
Docker base images often don't run cron. You must install cron and start the daemon (e.g. cron -f as the container's foreground process) — nothing schedules jobs otherwise.
FAQ
How do I check if cron is running?
systemctl status cron (or crond on RHEL). It should say active (running). If it's inactive or dead, no jobs will fire.
How do I make cron start at boot?
sudo systemctl enable --now cron starts it immediately and enables it for every reboot. Use crond on RHEL/Fedora.
Why did cron stop working after a reboot?
The service was started but not enabled. Run systemctl enable cron so it launches automatically at boot.