How big is your map?
The commonest reason for the raster export to fail is the IMC running out of memory. Are you running 32 or 64 bit Windows, and how much RAM do you have?
Whilst the map is generating, you can bring the IMC to the front (by clicking on the IMC icon in the taskbar), and in the IMC you can toggle to the status window and get some information about what is going on.
One thing I would try is creating a small map (maybe just a subset of your whole map) and exporting that. That will be (relatively) fast, and should help to rule out memory related issues. If that fails, then something else is going on so please post your results.
Matt