There can be many reasons your computer wont start up. A lot of computer companies will want to just replace your hard drive as a quick fix without really spending the time working out what is exactly wrong.
The problem with this is you will usually loose all your data and it is really only treating the symptom rather the real underlying problem, which can occur again. Below is a list of common problems why your computer wont start up.
Bad Software and Updates
You might not realise it but you have many different kinds of software and programs on your computer.
Some are very basic and do things like your calculator, others are a bit more sophisticated like your Office suite which does emails and word processing, some help your computer components like hardware talk to your operating system which are called drivers, and then you have the operating system itself like Windows, Linux or OSX.
These software manufacturers are always issuing updates for their applications. Most of the time these updates work seamlessly, but every now and again they cause problems on your computer from not working properly to your computer not starting.
Loose Cables
Yes, sometimes it can be something as simple as soon loose cables.
Your computer has a lot going on inside under the hood. There are many cables which join all your components together, specially through your motherboard. Your hard drive which contains your operating system and hosts all your files connects to the motherboard and power supply, which means it has two cables. Sometimes moving your computer or looking under the hood can very easily disconnect these cables which will stop your computer booting up.
This can be very diagnosed and happens a lot more than people realise.
Power Supply Issues
Power supply Issues are big problems in both computers and laptops. Your computer usually only has one PSU (Power Supply Unit) but it is responsible for supplying power to all your computer components such as your hard drive, screen, graphics card, motherboard and sound card.
If your Power Supply Unit it not working properly sometimes your computer will start up but wont function properly, other times your computer will make a noise, but most nothing will happen at all.
Its not just your Power Supply Unit that can cause problems with power in your computer. If one of your devices, external or internal, is not working properly it can send power back into your system and destroy other parts, which can stop your computer from working properly or posting, starting up.