Sometimes Descent 3 doesn't terminate its process after closing the game. Before launching the game again (without restarting the computer beforehand) make sure the MAIN.EXE process is not running in the background
To do this you need to enter the Task Manager. You can do this by right-clicking your Start Menu bar and clicking Task Manager. Click the Processes tab and look for the MAIN.EXE process on the list.
If you can't find it, then you can close the Task Manager and launch the game - it should start normally.
If you find it, however, you will need to terminate it manually before launching a new game. To do this you must click the process on the list, click the End Process button and confirm by clicking Yes. You can now launch the game normally.