How can I open an AutoCAD backup file?

If you have set Acad to save automatically every few minutes, you will find the latest saved file with an extension such as *.sv$. It might be called auto1.sv$, auto2.sv$ and so on. Those files are frequently found in the windows/temp directory.

Change the .sv$ to .dwg and you should be able to > Because an internal error, I lost an AutoCAD drawing, however, I have a

Yes, but I have R14, so cannot comment specifically on versions above that!

The important bit is the extension .sv$. Search for that and they can be found :-)