We have an endless series of posts about losing files and not having stuff backed up. Its important to have multiple copies of your work esp when you have invested time and effort into a file.

Its a good idea to save your file as Filename_001.mdr then next time save it as Filename_002.mdr and so on. Keep all of the files and maybe delete the ones 10 numbers back from the latest but better still to keep them all until you have completed your project and have it all finished to your complete satisfaction. And now and again back stuff up to an external USB hard drive. On windows you have xcopy still hanging about and it will do that backup for free. Hard drive space is so cheap these days and even if that is too much then get an SD Card which you can get about 64Gb now for about two beers.