Link to homepage of ONE subsite broken

So I don't know whether here is where I should be posting - but I need some help - I'm stuck ...

My site has been fcoming along swimingly ... however I've just discovered that the link to one of the subsites is broken - just takes me to a page of:

Index of /kingbilly

Parent Directory
Apache/2.2.22 (Unix) mod_ssl/2.2.22 OpenSSL/1.0.0-fips DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/ mod_jk/1.2.32 Server at Port 80

however all the links within that page (eg. works fine - it's just the homepage. All my other subsites work fine.

I'm halfway through setting up the global category stuff as described here -

I had gotten up to the "Step 3. Remove the +Add New Product Category link" and had installed those files. It appeared to work fine but it's just now I've realised that the link to the subsite homepage wasn't working. I don't know if that's relevant but thought I'd put it in just in case.

Wordpress version - 3.3.2

I find it weird that it's just one subsite that's broken.

Anyone have any idea what I've done and how to fix it?