Hey Terry
Importing a DRT file adds that timeline to the project and opens it in the Timeline panel, it doesn't remove any existing timeline. Simply double click the original/target timeline (in the Media Pool panel) to (re)open it in the Timeline panel. As John hints at, you might also consider enabling (Timeline View Options) "Stacked Timelines" to keep multiple timelines open in tabs, side by side.
BMD has some great free training resources available... its really worthwhile.
Check out the Beginners Guide
here.
Cheers
Andy