This is a common question I get asked, so here is a generic answer that will apply for future reference. If you get blocked it will be one of the following reasons:
1. Your IP address has recently been used to send SPAM - either on our server, or another server. We use "stopforumspam" to help identify spam sources, and block them. If your ISP changes your IP address and it was recently used by a spammer, this can cause you to get blocked. One other possibility is you might be compromised and have malware that is sending SPAM. Go to https://www.stopforumspam.com/removal to check your IP... and follow the process if it is listed.
2. Your IP address is being used by multiple users within a 24 hour period. Some annoying people create lots of fake accounts, and try to bypass limits. This restriction may block you.
3. You look like a bot. Some referrer strings will get you blocked. This is very rare for human users.
4. You trigger our IDS system. This is a lot more detailed, and will block known attacks. There are many rules that make it up, so I cant help you specifically - but thats the only other cause.
So. What do you do when you get blocked? WAIT. It resets generally every 24 hours, with the exception of stopforumspam. That will last longer, and removal might help.
CZ