PC doesn't boot

Solution
Start at the wall checking for power. PSU switched off? Open the case. if there's power there should be an LED glowing somewhere on the MB. Unplug and replug everything. Are there any LEDS lit up on the front? Dell has a comprehensive diagnostic LED array on almost all systems. Check the online manual for your specific Dell. It would help if you supplied more information about your computer than just "Dell".
Start at the wall checking for power. PSU switched off? Open the case. if there's power there should be an LED glowing somewhere on the MB. Unplug and replug everything. Are there any LEDS lit up on the front? Dell has a comprehensive diagnostic LED array on almost all systems. Check the online manual for your specific Dell. It would help if you supplied more information about your computer than just "Dell".
 
Solution
Inother possibility is a failing motherboard battery. This usually shows up as needing to rest the clock a few times. Replacing the battery will also reset the BIOS to defaults. There should be a jumper to do this also. See the manual for where and how to do this.