Disconnect the following from the computer and see if it at least powers on:
Any external devices connected to the computer, including the mouse.
Disconnect the HDD and the ODD (Optical disc drive) from the motherboard, both power and data.
Remove the RAM modules from their slots.
Try to turn on the computer.
Hopefully it will start and you will get beeps indicating failure to POST due to no RAM.
If it still doesn't start
Check that the following are connected properly:
On/Off Switch connector
System Board Power Supply connectors
Here is a link to the service manual. (tick the I'm not a robot checkbox , click Get Manual in Green box, pass the verification test and then click on the Download PDF or View in Browser above the Save to your collection box
It shows the necessary pre-requisite steps and the procedures to remove and replace the various components.
If it still doesn't start you may have a motherboard problem.
Hopefully this is of some help