I exported a wav - but I exported it and when it asked me to save, i replaced the file that i was actually exporting from. Then the wav is silent and I cant get it back? I have a wav sitting here thats 500mb but just complete silence.
Is there any way of getting the sound back?
Did you save the Audacity project?
Did you close Audacity, without saving the project before checking the exported file?
Did you “clean up” (delete or move) any files or folders before you exported the file?
I didnt save the audacity project, yes I closed audacity. I just assumed it would work.
So what is this 500 mb file of silence I now have?
Can the sound be retrieved from it?
It’s a computer. I’ve been working with computers since the mid '80 and they rarely “just work” (not even Macs). 
It’s a large, silent audio file. I very much doubt that anything can be retrieved from it, but what I’m more interested in is why this happened. If we can find out what went wrong then hopefully we can prevent it from happening again.
First, a bit about how Audacity works:
When you import a file to edit, by default, Audacity does not act directly on that file, but makes a copy of the audio data and works with the copy. The copy is initially stored in Audacity’s “temp” directory. If you save the Audacity Project, then an “Audacity Project File” is created (which has the file extension “.aup” and the audio data is moved to a “Project Data Folder”. So, for example, if you save a project as “test”, then a project file “test.aup” will be created and a data folder called “test_data” will be created.
However, if the file that you import is an “uncompressed” audio file, such as a WAV or AIFF file, then there is an option to override the default behaviour and make Audacity work directly with the original file. Doing this can make very large files import more quickly. Audacity will then save any modified data to its temp folder as required. You need to be careful when using this option. If you move, delete or rename the original file outside of Audacity, then Audacity will no longer have access to the original audio data (the original file). More about this option here: Audacity Manual
So now, going back to your silent show. Can you describe in more detail what you did that lead up to the problem?
Can you make the problem happen again?
Oh, and, what version of Audacity are you using (look in “Help > About Audacity”)
and which version of Windows?