Don't despair - there are some very good guys on this forum who can help. First thing, don't play with the computer in the hope of solving the problem - you may only make it worse.
What the experts will need is information about your system, what happened to it and whether there were any symptoms or changes before it all went wrong. So sit down, compose yourself (not easy, I know!), and jot down on a piece of paper
- a brief description of the essential parts of your computer, eg operating system, amount of RAM, microprocessor etc
- what happened when it failed, in detail
- were there any error messages on the screen?
- were there any preceding symptoms, odd events etc
- had you changed the system in any way before the failure eg added new hardware, installed new programs and so on
Remember, the more relevant information you supply, the more likely it is that they can provide effective assistance.