My Laptop Keeps Turning Itself Off Before It's Even Loaded Up?

4 days ago I turned my laptop off for the night as normal. When I tried to load it up again the next morning it switched itself off before it had started up. I have tried running startup repair but it always switches itself off before it has a chance to actually repair the problem. I have also tried starting it up normally, at which point it checks for inconsistencies on my hard drive and tries to fix the problem, but again it never gets very far before it turns itself off again.

It is a Sony Vaio laptop. When it turns itself off it makes the sound that it would usually make if you forced it to shut down via the power button.

I have racked my brains but can't think of anything that could have caused this problem. I have not recently downloaded or installed any new software, or installed any new hardware. I have antivirus software on my computer and always scan anything I'm thinking of downloading, I haven't had any warning messages when visiting websites. My antivirus program scans my computer every day and didn't find anything the last day it was working properly. I haven't dropped my laptop or knocked it, I just can't think of a solution.

I am writing this on a relative's computer, but it is very important that I get mine working again. It is my only computer and I am starting an Open University degree in a few weeks, so it really is vital.

Any help or advice you could give me would be greatly appreciated.

Thank you so much,

Lexi.