Actually, the "Not responding" message is a standard Windows message for all Windows programs that appears as soon as the program is busy for whatever reason and cannot be interacted with. You might have experienced it with other programs on your computer as well.
With TM Master it can occur when the software is retrieving large portions of data from the database. So when you for instance, click on a vessel name in the PO module to show all 13 000 orders for that vessel, this might take a couple of seconds to retrieve from the database. While the data is being retrieved, you might get the message "Not responding".
Usually just waiting for TM Master to finish retrieving the data will solve the "issue".