The answer to this lies in the fact that due to what reason you are blocked from accessing that particular website. It can be classified into two categories. One in which you are not given authority by the website to access its content while the other reason is that there may be some issue between the website and you. Each has its own set of the solution.
How to unblock the website that blocks you
This is because the website might have blocked a range of IP address or yours alone. If you think that you need to get out the problem then the only option that you have in your hand is to change your IP address either temporarily or permanently according to your need.
How to unblock the website that denied access to you
The primary reason why you are blocked by some web sites is due to the fact that your content filter will not allow you to access these kinds of the Websites. These kinds of content filters are popularly used in business, school and in some public networks.
These filters will not allow you to access the website which is considered as undesirable by these filters. This is because the administrator in charge does not like you to access this website or see them or they may contain things that are not suitable for you to see.
One way by which you will be able to overcome this kind of block is by using the anonymous proxy. To know more about these you can visit them in any of the proxy Websites list that you can find on the internet.