The 403 Forbidden error is an HTTP status code that means that accessing the page or resource you were trying to reach is absolutely forbidden for some reason. In other words: "Go away and don't come back here." You can solve this issue by cross checking your specified URL.
Obviously it will effect, if that page is ranking for a particular keywords then ranking will go, as Google will not see the content on that page.
The web server may return HTTP Status 403 Forbidden code in response to a request from a client to a Web page or resource to indicate that the server refused to allow the desired action. In other words, you can access the server, but the server refused to allow the request.
We can avoid such type of effects by using 301 redirect. With the help of 301 redirect, you can redirect your forbidden pages to some other pages which are ranking good in Google or any other search engines.
A web server may return a 403 Forbidden HTTP status code in response to a request from a client for a web page or resource to indicate that the server refuses to allow the requested action. In other words, the server can be reached, but the server declined to allow the requested access.
It affects overall SEO of your site so you should redirect them on any relevant page or home page, make sure that you are doing SEO friendly redirect, i.e., 301.
I think you should make one great 404 page . if customer put wrong URL or reach old remove page so it will be transfer your 404page . You can get guide from Google how to make 404 page .
It's possible that your public IP address, or your entire Internet Service Provider, has been blacklisted, a situation that could produce a 403 Forbidden error.
403 Forbidden error shows up if there is a problem with the .htaccess file. If the passwords are changed properly this error is likely to show up.