Hey,in order for the forumers and moderators to help you with your problem,post the information required about the PC/Hardware in question here:

What is your:
CPU: (Intel Core i5-2400 etc.)

GPU or Graphics card: ( Nvidia GeForce GTX 460 etc)

Motherboard : ( Asus P8P67M etc.)

RAM : ( Kingston HyperX Fury 2x4GB 8GB DDR3-1600 etc)

x number and brand of Hard Drives/SSD: ( 1x Western Digital Caviar Black 500Gb etc.)

Computer Case: (Cooler Master HAF 932 etc.)

Power Supply : (Corsair CX430 etc.)

(Insert audio card here if applicable)

(Insert network card here if applicable)

What is/are the problem/s of your PC/hardware?

What steps in troubleshooting did you do in order to diagnose/reproduce the issue/s?

Have you done any Hardware/Software changes before the problem became apparent?

What was the event/s preceding before the problem appeared?(knocked over,power surge etc.)

These are some of the actions that you can take in order to narrow down/solve the problem:

Reseat or Reconnect all cables/connectors/hardware inside the computer
Many problems can be pinpointed to cable connection issues. Sometimes,they are inadequately connected. Sometimes they are excessively connected. Either way,reconnecting cables one-by-one helps to eliminate bad contact points among hardware.

When the PC boots up to the Operating system but fails,it is more likely defective/corrupted drivers and/or software causing the problem.
In order to solve the specific software problem, boot into rescue/safe/recovery mode of your Operating system, uninstall the offending software component, and reinstall an older or newer version of the required software component depending on your usecase.

Then,If it refuses to go past the BIOS screen,or if the display is completely blank,it is more likely hardware is the source of the problem.
In order to alleviate this situation, Reseat or reconnect one component per instance and try to boot. If it gets stuck when you find the offending pice of hardware, either fix it in BIOS or Operating system settings that govern the operating state of the device or replace it when it is defective.

Look for possible short circuit points in the computer case as it may be causing Short Circuit Protection of the power supply to activate and shutdown the power supply every time you try to boot it up
Prevent short circuiting by organizing connectors/cables and ensuring no loose connections to hardware.

If your motherboard has a Debug LED,look on what code it displays when the PC does not boot up and look for the code in the motherboard manual.
By doing that,you save time by knowing the some or all elements of the said problem and can find the solution quicker due to the localization of the problem scope.

When you have driver problems,use Driver sweeper for general hardware and Display Driver Uninstaller to uninstall the current drivers and clean install the replacement drivers required and to prevent driver related problems
By following this step,you are eliminating Many driver problems outside of driver bugs and incompatibility.

When software behaves erratically,rebooting the computer usually solves the problem.

Thank you for reading this,but if the problem/s persist,make a thread here with all the relevant information required so we can help you solve the problem.