Hi Rainbow Dash, I've spent a few hours in build mode also today tryng to figure out what might have gone wrong. I couldn't replicate what happened to you without going into live mode to 'look' at the build and direct my sims around etc or by not hitting pause (like PaisleyByrd stated).
I'm not sure what to suggest other than save your build to your library when you get it completed and save your sim. (Your sim will already be in your library so you'd only need to save her/him again if there was any skills gain.) Then start a new game, download your house and enter live mode and take it from there. Least that way you will still have your new house and yet your game will be back at the beginning of the day.
