I am working on a unity project that is a very big project but unfortunatly my game has been crashed 2,3 times without any reason..My 2 months hardwork has been wasted.. Plzz help me why this happens on unity..I am using unity 4.3.4

Ok take a deep breath:)
There could be many reasons behind this.
First of all, make sure that all scenes are not opening. If any scenes open that means you have recovered them, and saved part of your undoubtedly hard work Scenes can be accessed from the project folder>assets.
Maybe a corrupt script could be the root, you can temporarily remove the scripts from the project through the assets area.
A possible cause is hardware problems, make sure your computer is capable of running such a big project, and has enough space for it.
If all else fails, attempt moving the project to another computer. This link might help moving it: http://answers.unity3d.com/questions/273912/moving-the-project.html
I reallllllyyyy hope you can restore your files :) :) :)