If, out of the blue and with no changes to your site, you are suddenly getting 403 errors, it may be that your ISP has assigned you a new IP address, and that IP address has been listed on the Composite Blocking List (CBL) for some sort of abusive behavior. Our apache security module will deny requests coming from IP addressess listed on the CBL.
Here's how to determine if this has happened:
1. Get your internet IP address at http://www.whatismyip.com or any site which displays your IP address.
2. Copy that IP address and submit it at http://cbl.abuseat.org/lookup.cgi
If your IP is listed, at the bottom of the page describing why, you can have it delisted. Only do this if you are certain that your system isn't compromised somehow and causing the abuse listed in the report. If the CBL reports activity in the last 24 hours, you may actually have a problem on your network that you are unaware of. If the reported last occurrence is days ago, it is more likely that your network is okay.