The problem is you had either a view activated or a the sheet activated.
Simply click through the views, doing an RMB - "Lock View Focus" and if the sketch line turns [blue]blue[/blue] you have found your the view. If none of the views work try doing a "Lock sheet focus" to see if that gets it. I can only be one or the other, there is nothing else that would keep a sketch line from being inactive.
If you don't get the option "Lock view focus or sheet focus" see faq559-1018 or check to make sure that Tools\Options\System Options\Drawings\ "Dynamic Drawing view activaion" is turned on. If it's unchecked that's fine. You can simply Double click each view to activate it instead of RMB each view as mentioned above.
If decide to use the Double click to find your views or sheet. You still must follow the instructions above.
