Usual thing of did you email it to someone, stick it on a USB drive, commit to another source
.a files might be able to be extracted with the AR command
http://sourceware.org/binutils/docs/binutils/ar-cmdline.html
What you might get back from it I do not know but it unlikely to be the whole file.
Strides are being made into C and C++ decompilation but it is not really something I can point you are like I might for Java, C# or similar languages. Even with such a thing your comments, labels and other things are not going to come back.
In any case it will probably be easier to remake it. Hopefully you can remember the broad strokes, and maybe have something jogged but what you have remaining.
Also "backups, keep them". This is your lesson in it.
Equally you might want to consider starting to kick things to other files -- I usually go with if the main is more than the start, libs, calls to whatever and exit routines it had better be an extremely simple program.
It is odd that notepad++ did this though, while it is far from stable at times this is very new to me. Are you using some odd plugins?