Hdmi no signal

Hello,
Upon starting up the computer, I get a message showing HDMI No Signal . Then the monitor will go into sleep mode (light turns from blue to orange). Also, the USB keyboard and mouse are also not being recognized. Seeing I have to onboard graphics disabled I cannot revert to the default VGA. Plus, without an active mouse and keyboard I could not change settings anyways. I have tried to take out the battery and clean it, tried replacing the RAM and also tried connecting a PS2 keyboard to no avail. I have a HP D7800 with Windows 7 Home Premium 64 bit. Additionally, I can plug other devices into the USB ports such as a tablet and phone, and the ports work with those as I get a message showing the drive disconnected as a media device. Any help would be appreciated.