What do you mean by overwritten?
Do you mean that you opened the txt file that had the info on it, then typed something out and deleted the original text and saved the file with the original file name?
Or, just deleted the file, then emptied the recycle bin?
Before you do anything, do a search for part of the sentence that was deleted under the 'find string' - F3 button you might be lucky and the file was temporarily saved.
otherwise:
If the first, you may have a problem. - you probably won't get the original text back, but you might be able to salvage it - LOTS of hard work and if your computer has been on since you deleted the message, it might not be there anyway.
You'll need to get a hex editor which can look up a particular sector [512bytes] on hard drives - then you look through the whole hdd - hope its a small hdd with a very small or fixed SWAPFILE/pagefile. This will take you a few years. You might be able to find some software that will do an automatic search for a string text.