I have the same problem all the time. One tip though, if you have been building for a while - no set time just when you've done a few things you wouldn't want to have to redo - SAVE the game. And occasionally SaveAs with a different name in case the game gets corrupted and you lose it all.
You have to go into Live mode to Save.
If the interface locks up - use the keyboard that usually solves the problem.
F1 key = Live mode
F2 key = Buy mode
F3 key = Buld mode
If things are locked up don't worry about which mode you want to be in - just get into another mode. Once you fix the locked up interface problem you can save the game and go into the mode you want to be in.
Another good combination is Alt key + F4 key. This will shut the game down and it will usually ask you if you want to save before closing. It will only ask you if you want to Save is you are in Live mode.