I have the latest version - 10.0.4 of PLSQL Developer.
I started Extract User Objects, but in the middle of the process colleague restarted the database.
As a result PLSQLDEV shows it is extracting. It doesn't tell me there is a problem.
Also I cannot "Break" the process.
I cannot close the window, nor can I close the whole app.

We'll check it out.

I experience similar problems when the network link to the target system fails for some reason ( and not only for compare user objects) The only "way out" is to use the task manager to kill PLSD, of course by this loosing the unsaved window contents.

I've had a similar problem. Check your TNS definition - if it has something like "(RETRIES = 180)(DELAY = 5)", it can be contributing to your issue. (Why Oracle seems to like "RETRIES = 180" is beyond me; this causes programs (like PL/SQL Developer) to retry 180 times (with a delay of 5 seconds) when there is a connection issue.)

180*5 means that connections will seem to hang for at least 15 minutes trying to communicate with Oracle. If you leave PL/SQL Developer sit for 20-40 minutes, it will eventually time out. (You can also try disconnecting the network connection on your PC, which seems to allow SQL*Net to give up without retrying the full 180 times.)

I've changed my TNS definitions to use RETRIES=2, and I no longer have this issue.