Reaching the end of the queue does not mean Minecraft has started. If Aternos displays a confirmation prompt, confirm before it expires. Then check whether the state changes to Starting.
Follow the state shown in the panel
If the panel returns to the queue, check the confirmation and the message on screen. If Starting ends in Offline or Crashed, open Log in the panel. In the example flow, Online is the point at which players can try joining.
Waiting in queue → Confirm → Starting → Online
Find the first log line about a failing mod, plugin or world. Back up before testing a recently added component. Record the current software and world before using a reinstall option.
If the panel shows Online, test the current address and matching game version to diagnose joining separately.