Shutdown the power, disconnect all drives and all cards aside from the video. Disconnect power to the now idled components. Make sure there are no foreign objects or case parts touching the motherboard anywhere other than where it should be (the mount screws/retainers)
Try to boot and see if it will post. If it does, go through a sequence of shutdowns and restarts while connecting and powering one additional component between each startup effort.
You didn't say if you have a modem but when such a condition develops for no apparent reason and a modem is present - it's my first suspect.