If you have just changed your nameserver assignment from pointing to the old host to the new, give it several hours to a day to propagate throughout the Internet. I had changed my host once, however, and did not see the forbidden error. Instead, I saw my site being served from the old host until the switch was made. -- When the new host's site got wiped by a bot attack, I was down for maybe 3 hours while I waited for my reassignment to the original host to once again take hold.
Otherwise, there may be an issue with regard to your account setup being different from the old.
For instance, my original host gave me a temp address of hostdomain/myusername/, which was then replaced by my example.com/ when my domain name came into force.
Over at the new host, their address scheme was hostdomain/~myusername/ and I had as I recall, to change a bunch of paths from /css/stylesheet.css to /~myusername/css/stylesheet.css, before my domain name took effect at which time I could resume with the /css/stylesheet.css approach.
All of this meant there were errant paths and resources were not where they were expected.
Assuming that you left a host using Apache and are using a new host serving on Apache, make sure you have your .htaccess set up right, so that your example.com/ will bring up your example.com/index.html (or whatever name and extension is on your start page).
Code:
DirectoryIndex index.php index.shtml index.cgi index.html
This line will make index.php your default page, and if it is not there, the first name on the list to its right, that is present, will become the default page. And if none of these names exist in your directory, then there will be either a directory listing or a 403.
I covered a lot of possibilities, and hope I shed some light.
Larry
