Verify that your password is correct by logging in @ appleid.apple.com. If it doesnt work the issue is password related. If it works power cycle the apple tv and the router (powered down for at 30 seconds). If you still have the problem after that the issue could be port related. HT2463 lists all the ports that are required. If you don't know how to open ports on the router call the router manufacturer.
Home Sharing Troubleshooting article should cover all the basics as well.
HT2463 (ports): http://support.apple.com/kb/HT2463?viewlocale=en_US
TS2872 (troubleshooting homesharing): http://support.apple.com/kb/TS2972?viewlocale=en_US