I have used Audacity successfully on this computer, but it suddenly won’t record for me. I can see the waves as I record, but as soon as I hit stop, it flatlines and there is nothing there. I tried hitting pause to see if I could save the file, but the option is not available. I even tried uninstalling and reinstalling. I also tried different microphone settings. I am using Windows 7 Ultimate and have Audacity 2.0.5. I am recording with a simple microphone that plugs in to the microphone port on the front of my computer. It is not USB. This is the same equipment and software I have used successfully (in May 2014).
Audacity saves the audio data in a temporary folder, or at least, tries to. If Audacity is unable to write the data there, then the data will disappear as you describe.
Look in “Edit > Preferences > Directories” and check where the “temp” folder is. Is that a location that is writeable for your user account?