Question on saving

[*pinky*]

I am very frustrated about the saving with this game. I obviously dont understand it. I made an entire house which took days and i had a sim living in it with his career level pretty high. I went to go start a new game and when i left it...it asked if i wanted to save and i said yes, save. well when i went back in to play that sim and house i had been working on....it was gone. the sim is in the bin but the house is nowhere to be found. What am i doing wrong? Im ready to give up on this game....this is the second time ive been through this. Could somebody please explain to me on how to save so it wont over ride other game plays!! Thank you in advance!!