Someone has hijacked my site!

Hi.

I set the following site up about a couple of years ago. http://www.healthandfitnessreviews.info

I just went back to it today and have found lots of content that I did not put there, and obviously I cannot log into the wordpress module with the same log in information, and also he has changed the email records so wordpress has no record of the email I registered there with.

How do I regain control of the site? I am moderately IT literate (ie can follow instructions) but dont have a clue.

If the answer is that I need to delete the whole word press installation and start again that is fine. But I would quite like to gain control of the site like it is so I can change all the affiliate links to my own accounts! As he has been stealing from me

Thanks v much in advance, guys