Could it be that the theme is not xhtml valid?
No the problem is that the style sheet has been deleted either by you or by your host or by those wonderful and most loved people called hackers.
You say you are not a techie so may I assume that you haven't done any changes to your theme? If this is so, you can simply re-install the theme at the same location and everything will start working again.
If you have made any changes to the styles, they are all gone anyway and also, you need to find out why the files have been deleted. This would mean that you will need to change your passwords and all that security measures people normally take.
But first thing is to get your site working and so re-install the theme.
Hope this helps.