Issuing a d appl=(application name) may show a status of "inactive" and you won't be able to logon to it. There are some things you can do to activate the application other than recycling TPX.
Resolution
In TPXOPER issue an 'ACT applid' command to activate the application.
If that doesn't work then check for a hung terminal:
Determine what virtual terminal is being used to logon to the application.
Then in TPXOPER issue a 'MOD VT=vtermid,UNAVAIL' command to make the terminal unavailable.