I've now had this issue a couple of times. I execute a rather simple query and PL/sql developer seems to execute this and the execution timer walks, but on the database nothing seems to happen. There is a session on the database, which is not active. When I kill this session or press abort query the timer just keeps walking.

The only way to stop this is by force close PL/SQL Developer.

Certainty of death, small chance of succes,
What are we waiting for... (Gimly)