lighroom modification done on a raw file are no more on screen when i reopen the raw

Assuming you are using the cloud-based Lightroom (current version 4.1), you need to be aware that when editing an image the various edits that you apply are not synced to the cloud until you move onto another image. Normally, if you exit the program without "saving" those changes to the cloud (which is where the master catalog is), they should be saved as part of the application shutdown process. However, there's currently a bug which means the edits are NOT automatically saved if you exit the app before moving on to a different image. 

So when you next start Lightroom, those edits that you did will not have been saved, so you'll still see the unedited version. I assume the bug will be fixed in the next update, but in the meantime I suggest you try to ensure that you "save" any outstanding edits by moving to another image before you exit.