I have encountered this error before and it has just occurred again. QGIS crashed before my project could be saved. When I attempted to re-open the project I got the same error as above. The project file was empty (0k).
I found the actual project data was stored in a file (which may be hidden on your system - if so, you need to enable view hidden files) with the same project name, but called
To recover, I opened the
.qgs~ file in Wordpad and then saved it immediately as the .qgs file, overwriting the empty
.qgs file. I closed Wordpad, launched QGIS and reopened the project - no problem.
Using Windows 7 home 64bit and QGIS 2.4.0 64 bit.