You might have hidden it behind a panel or something.
You could try the following:
- Click the TVirtualStringTree in the object insprector
- Richt click on TVirtualStringTree
- Choose Z-Order and then "Move to Front"
Is your TVirtualStringTree now visible. If so, you need to rearange some things or make sure TVirtualStringTree is a subcomponent of the panel under which it was hidden.
If it is still not visible you could check the Left and Top values in the object inspector for that TVirtualStringTree. Set them to 0 and try above again.