If your IP address was banned from particular website,you are unable to use that website again.Also you are unable to post.It happens because if you are violating or against the website rules.For this reason,the administrator will ban your IP address at any time.You can contact the administrator to get unban your IP from that particular website and ask him the reason why he was banned your IP address.Otherwise try to change your IP address and then visit that website again by creating another account.
Follow the below steps for changing IP address in Windows XP:
1.Start->Run->Cmd->Press Enter
2.Type in "ipconfig/release" for Current IP address and
3.Then type "ipconfig/renew" for New IP address .
You can also change your IP address by this way,
If it is static IP,you cannot change it.Static IP will not change even when you reset the modem.You have to contact your Internet Service provider(ISP) for changing static IP.If your IP address is dynamic,it will change whenever you reset the modem.You can check your IP address by using Ip-details.com .