First off, run the render diagnostics.

Rendering menu set>Render>Run Render Diagnostics. Fix anything that it reports in the script editor.

If it still doesn't work,

Rendering menu set>Render>Render Current Frame>Option Box. Change the verbosity level to detailed messages, then click render. Check the output window (I'm not sure if it still exists on Windows) or the Console on OS X, and see if it reports any errors there.