[System.FormatException: file is not a valid paint.net document]

This is extremely furstrating.

[System.FormatException: file is not a valid paint.net document]

I didn't manually do it.. in fact, I don't know how. I chose save as, then drop down to JPG... and this is what happened. 

[System.FormatException: file is not a valid paint.net document]

It does open, but why did that happen? I have done this process multiple times. Where is my PDN with layers now?

[System.FormatException: file is not a valid paint.net document]

I need some help... Does anyone know why my PDN is giving me this error? I just finished a small project. Saved the PDN. Then I saved as a JPG. When I went to send the JPG, it did not exist, only the PDN existed. I then tried to open the PDN to resave as JPG and now I get this error.