Drone CI comes since version 1.5.0 with multi-server mode (in which builds are processed by external CI runners) activated by default.
This can be problematic if you're used to drone running in single server mode.
If single-server mode isn't explicitly disabled and no runners are configured it just causes your builds to be stuck with a
It took me quite a while to figure why drone suddenly stopped processing builds (to be fair I updated without reading the changelog).
Luckily there is an easy solution to re-enable single-server mode.
Just add the environment variable
DRONE_AGENTS_DISABLED=true to your drone instance and it should process builds again like it did before.