Ok well the deal is my Simself was engaged and had a toddler. Now all of a sudden she has moved away without her child or fiance. I really want her back. The whole story progression thing has made me a bit mad but I've dealt with it. This is unecceptable though. Is there anyway to get her back? Cheats or anything I dont mind!

I think you can turn off Story Progression from the options menu, at least that is what I have heard.

But, as for what happened...I have not personally had this happen but I think they just move into a random house in the neighborhood. So, look around for a bit and in the families bin too. Once you find her you can evict her and then merge her back with the family that she came from.

Edit: Apperently that is not quite what happens. It seems that the sim vanishes entirely. But, there is a chance that they will come back. If that happens, make friends with them again and just do the above if you can find where they are living...

Thanks for your help but she is definitely gone completely. She is still shown as alive and well on the family tree but she is nowhere is anyones contacts lists.

I just updated my reply. But, there is also a feature that automatically saves a copy of every sim you make into the Sim bin it seems. So, if that it still there you can just place her down again..but it means starting over again I am afraid...