Sometimes I get a similar issue to this and need to remove the CMOS battery and put it back in. Another thing I have tried in the past with pretty decent success rate is removing and reseating the ram. I don't know why that fixes boot issues but it does. First thing to try though is unplug from the wall and run on battery then hold the power button while the computer is booting or trying to for about 10 seconds until the computer powers completely off. keep holding it for another 10 seconds then power it back on. That will drain all the power out of the computer and allow it to boot cleanly