Unable to "Wake up" computer from sleep mode

Periodically I am unable to wake up my computer after it goes into the sleep mode. Pressing the space bar or another key does nothing. I have to push the power button to turn the computer off. Sometimes when I turn the power back on, nothing happens - no boot-up. If I turn if off a second time, wait a few minutes, and turn the power back on, it reboots with screen display "Resuming Windows" but takes a long time to reboot. I have a Desktop Dell Inspirion 518. Can anyone tell me how to resolve this problem?

Thank you very much!