Hi,
What is wrong with this sentence? "My computer was accidentally shut down".
Actually, there's nothing wrong. It's just a passive sentence. You'd see that more if you said My computer was accidentally shut down by Tom.
But I don't think this is what you mean. I think, as has been suggested, you mean your computer crashed. You mght also say it 'froze up' or 'locked up' if everything on the screen just stopped and you had no cursor.
Best wishes, Clive