1) Is there any specific reason you installed DD-WRT? Did it add something you needed?
2) Is the problem on ALL PC's on your network?
3) Since the iPhone doesn't work either, it pretty much has to be something in the router. I know it is a pain in the backside, but you can backup your configuration (like the previous poster recommended) and reset the thing (plus DD-WRT may need an update).
4) Reset it - then take things one step at a time back to the setup you need.