First of all, you need to login to the google webmasters to identify the errors.
There are majorly two sections
1. Site Errors and
2. URL Errors.
You have to figure it out first what type of error you are getting. And for that, you need to go to the Crawl Errors section. In the Crawl Errors section, you get the complete list of errors from which you can easily identify what type of errors you are getting. That means whether they are Site errors or URL errors.