There are two common types of "not starting" problem with a desktop PC - either there are no signs of life at all (no power light comes on, no sound of spinning fans) or it starts but then either resets itself or "gets stuck" before it has fully started.
If there is no activity at all, you can check the connections yourself. Make sure the power lead is firmly connected to the computer and the other end is securely in the socket. If you are using an extension socket, check this too. It is also worth checking the fuse(s) in the plug(s). If these checks don't help, the problem is most likely to be in the internal computer power supply unit or the main circuit board. We can arrange an engineer to test these components for you.
If the computer is resetting or failing to fully start, the cause may be a faulty component or hard disk corruption. You can try to start the computer in "safe mode". If this works, you can then check the hard drive for faults, which Windows may be able to correct, allowing you to start the PC normally. If the computer will not start in safe mode or the disk check doesn't fix the problem, our engineers can advise on how to fix the problem.