Tortoise SVN does not ask for user/pass and fails

403 Forbidden is one of the HTTP Status Codes. As per the definition:

The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.

I imagine that your repository is something like http://server/repo/project/ so try browsing that URL with your regular browser. Maybe is not related to Subversion authentication but HTTP authentication.

I know it is not a good answer, but it might give you a hint to expand the question :)

It's the authorization that failed. That means the first step, the authentication was successful.

So: the username/password is correct and was accepted by your server. But then that user wasn't allowed to access the path/resource.

check your path based authorization file (conf/authz) and make sure that user has the rights.