I have Dell optiplex 780 desktop, with new battery, beeped 6 times when starting, but didn't boot up, screen signal says check cable. It works fine when used on an XP older dell. I removed 1 card, restarted,  no beeps, but it didn't boot up. I put card back in, started computer, no beeps but it failed to reboot, I restarted it and pressed f12 repeatedly, but it hasn't rebooted. In the past if not started for a few weeks it always took ages to boot up. 

Thank you for all feedback.