power on problems....Have checked obvious!

Without knowing what exactly you've tried, I'll probably suggest a few things that you have already tried.

First, are you sure the outlet works? Try connecting a simple device, like a lamp or radio, and ensure that the outlet is okay.

Second, try reseating the power cable. Also, bypass any surge protectors, power stations, line conditioners or UPS systems.

Third, try using the monitor's power cable (it will fit and work just fine). If it works at this point, you probably have a defective power cable, and should contact Dell Technical Support for a replacement.

Fourth, make sure that no one has altered your voltage input switch. With the computer unplugged toggle the switch a few times, and (if in the US) make sure that it is set to 115, and that the switch is as far over as you can set it.

Fifth, move the computer to another room, and try a different outlet.

If at this point you are still getting now lights, and no power we need to open up your system unit. Make sure, inside, that all connectors from the power supply to the motherboard are firmly connected. With the system unplugged reseat each connector. There should be two, depending on which Dimension system you have.

If you are still not getting power, I would recommend contacting Dell Technical Support. It is likely that you need to have your power supply replaced.