How to 're-dock' the Viewport Tab

Some how my main window viewport became undocked and I cannot get it redocked no matter what I try.....this is very frusterating and I am quite sure that it is something simple that I am just missing.
Can anyone give me some ideas?
Comments
Make sure that Docking/Undcking is not locked in the Window>Workspace menu. If not, drag the title bar (not the tab) of the pane until you see a line in the highlight colour in the space you want it to occupy and then drop it - drag the tab if you want to add to an existing pane group.
Yep, have done all that. Only place I am able to dock is horizontaly above other tabs not between them....Ended up uninstalling and re-installing Studio to get it fixed. Not sure what the issue was but got so frustrated withit I re-installed and started over.
Thank you Richard for the help.
Reloading your layout (Window>Workspace>Select Layout) should have worked without the need to reinstall - not sure why I didn't mention that above, I usually do.