This issue is a rare one that pops up for people using Multiple Screens. It's hard to locate what the issue is and will happen on a working version of Dimensions.
If you have this issue then please DO NOT Uninstall Dimensions until the following has been checked:
Ensure all Screens are set up.
Click Dimensions in the bottom window so the window is Open but you still can't see anything
Press ALT + Space keys together. This should display a Maximise button in the top right of your screen.
Click the Maximise button.
Dimensions should now show up and allow you to log in as normal.
If this still has not worked a re-install may be required.