Dell Dimension 5100 won't start after power failure. Monitor and speakers ok, but tower won't power on. Ideas?

Probably took a power surge, more likely when the power came back then when it went off. (Always unplug your PC in a power outage.)

Probably damaged the power supply. Fortunately, the power supply in a Dimension 5100 is a standard size, and can be replaced easily. Price should be in the $40-75 range, depending on the units wattage (sorry don't know it off the top of my head.)

If the unit is still under warranty, contact. Dell before doing anything. They may replace for free.

It is also possible that the surge may have damaged more then just the power supply, in which case you may be looking at profession help or replacing the PC.

You can get a PSU tester to be sure. I had a dead 5100 in which I just replaced the PSU. Although the connectors match just fine, not all generic PSUs will fit. If the power outlet is on top and too close to the edge, the PSU wont fit. Rule of thumb is that the PSUs with power outlet without triangular mounting points on each side should fit. This is the power supply I used. I even had to return few PSUs to ensure that I get one with proper power outlet location.

http://www.microcenter.com/product/367692/Gold_Ser...

Could be anything, do you get any activity at all when you press the power button? If not then your power supply is probably dead.