This answer will give you several tests to try before contacting 123-reg support with web forwarding problems.
Firstly, make sure the website you are forwarding to is working
If you have just initiated the web forwarding, please allow a couple of hours for the update to take place.
Check that your domain name is pointing to the correct IP address in the DNS settings. To see how, please view the guide How do i set up DNS for Web forwarding?
Check that your domain is on the 123-reg nameservers by looking up the name on www.who.is.
Check that the URL is correctly entered in your 123-reg control panel (123-reg.co.uk control panel).
If the forwarding has stopped suddenly, make sure that your domain name has not expired in the 123-reg control panel (123-reg.co.uk control panel).
If none of these tests provide a solution, please contact 123-reg support.