I bought this used I believe mb is upgraded to asus it is hp case and hdd which has failed and I'm replacing it with no luck, the system has stopped posting, it was until I changed hd , it won't post with old hd, or without, I have the case open drive is accessible. thanks
Next steps should be to determine if the GPU is working. If GPU is ok, then it may be a motherboard issue.
Regardless of the of HDD if it is connected or not, the system should still post. You may also try defaulting the BIOS or try locating the BIOS reset pins or remove the mobo battery and then try rebooting.
Silly question, but is power supplied to the mobo and does the power supply fan spin up when turned on?
That leads me to believe it's the motherboard, may be something got shorted out while installing the new drive. Is there a way you can test all components on a different board? At this point thats the only thing I can think of.