[SOLVED] My theme and website has dissappeared completely after upgrade and change of

I upgraded to the latest Wordpress available and everything was fine, then i noticed there was another atahualpa theme listed in my theme options and for some reason,(please don't ask me why?) i clicked activate.

When I clicked on my site I got a load of warning messages but now I can't get onto my site at all.
The site took me ages to set up as i am not very technical and its so important as its for my wifes fundraising raising money for cervical, Breast and ovarian cancer.

Any ideas at all really appreciated as just in a state of panic at the moment and not a clue what to try. if i could get onto the site I assume I could just click on the original Atahualpa theme and everything would be great again?

Please help me if you can.

Kev