If phpBB's versioning server (http://version.phpbb.com/) is down or uncontactable (as happened on the 3.0.9 update), the ACP may fail to load on any page that relies on contacting phpBB's servers. This includes the ACP homepage where most users will enter the ACP.

Suggestion to fix:

Use CURL with a fixed timeout of 5-10 seconds so even if the server is down the ACP will eventually load after a graceful timeout.