A 403 Forbidden error is a webserver’s code for "You’re not allowed here."
Determining why
Is it just you that’s getting the error or is it everyone? Visit http://www.downforeveryoneorjustme.com and type in your website’s URL to find out for sure.
If it’s just you
It’s probably that you were mistakenly blocked by a Security measure that keeps your website safe from Hacker’s bots.