If you update PadCAD to version 1.5 there may be a small bug during the update process.
Fortunately it's an easy fix.
Just delete and re-install the app and it should fix your problem.
You won't have to pay of the app again once you delete it.
Apple keeps a record of all the apps you've purchased and lets you re-intall them if you delete an app.
Again, sorry for this problem but I have no control over the update process. It's something that is taken care of by the iPad software.
WARNING : Make sure you save any important drawings you may have onto your pc or mac via iTunes before you delete your app!