If possible, post the file (no copyrighted material, though!) - it wil probably be easier to help then.
It is important that you (if at all possible) find the errors one at a time, from the top of the file, as one badly placed or missing tag in the beginning of the file is able to generate two million error-messages (well, a lot
)
Another tip, which helped me a lot in the start: Try to unzip the epub and run the (x)html file(s) through the W3C validator at
http://validator.w3.org/#validate_by_upload - set the "More Options" to "Verbose output".
Hope this helps,
Regards
Kim