|There is no conversion possible from an exe to a jar file, at least not in anything like it's original form. The source code does not exist in the exe.|
This illustrates once again the importance of having a backup of important files. Hard drives fail, files can become corrupt or accidentally or maliciously deleted, etc. You must be prepared when (not if) this happens.
Having only one copy of an important file is asking for trouble and sooner or later you will get it. If the file is of particular importance you should have multiple copies kept in different locations.