I attached a picture as I can't figure out exactly what that is called. 
 
I can still click in a place and play or split clips but I can't see where the marker is at any moment. This is very frustrating. I don't know what I did as it was working just fine. 
 
When I click on a clip... instead of showing the vertical line.... it simply selects the clip. 
 
Here's the pic of the missing "Selection marker" or "vertical line" I'm referring to:
 

**Sonar X2** 
 
Thank you in advance!
 
Tom

That is the Now Time Marker. There is an old thread on that (http://forum.cakewalk.com/FindPost/2673327), and I am not sure if it was truly fixed with the X2a patch (http://www.cakewalk.com/Support/Knowledge-Base/2007013301/SONAR-X2a-Update) or not.
 
There are some workarounds in that thread, and the simplest is closing/reopening the track inspector (hit "I" twice).
 
Check that thread above out and welcome to the forums!

This really sucks.
 
I did all of those tricks and nothing brought it back. Working without the 'now time marker' is like driving blind. 

Save/Close/Re-open the Project should restore it. Otherwise restart SONAR. Other users have reported being able to restore it by changing screensets/views, but none of that ever worked for me. Haven't seen this since sometime in X3.
 
Not sure what all is in the referenced thread, but I recommend setting Windows Visual Effect to "Best Appearance" to help prevent this.

From what I see, Now Time Marker is still a separate window (!). In other words, graphic card drivers (and settings) as well as visual settings in Windows can influence it.