I’ve just had someone with this problem.  It is a fairly common situation, and mostly it is easily solved, but the steps are not obvious so I have outlined them here:

When you start a GNS3 Workbench exercise or lab, you should see two windows open, one is the GNS3 window, the other is where the Konsole application displays the instructions and the VPCs window.  Sometimes one or other of these windows gets hidden behind the other.

To successfully start a router console session, must must:
1. Firstly, ensure the routers are started – Click the Green “GO” icon in GNS3
2. Click the “console to all routers” button on GNS3 (the black icon to the left of the Green GO icon).
At this stage, your console sessions should start, but it may not be obvious
3. Press Alt+Tab to bring the Konsole session to the foreground.  You should see some tabs labelled R1, R2 etc (reflecting the names of your routers).  This is where your console sessions are.