How to undo after fixing my model

I saved my model then by mistake I fixed it ,I want to revert back …!!

Describe the steps you went through and what you have got now, (for it isn’t clear to me). So we can think with you.

What more have you done since you fixed it? Did you save it again? Did you close SketchUp? If you haven’t saved again and the model is still open in SketchUp you can do File->Revert, and it will go back to the last saved version. If you saved it again but only once, you can probably open the backup file that SketchUp automatically creates. It should be in the same place you saved the model file, and should have the same name with extension .skb instead of .skp.