I had been running a very out of date version of zencart (1.3.8). I know that I should have updated it ages ago, but..

Although very out of date, it worked, that is up until my hosting company upgraded their php engine. As soon as it fell over I asked them to downgrade it. Whilst the site still worked, it is no longer possible to log into the back end.

When I go to the admin folder nothing is visible, no log in prompt, nothing. The files are still visible via FTP, I can view the php files via the cpanel file explorer, nothing is jumping out at me as being amiss.

I have installed v1.5.0 in another folder on the server, transferred over the database and then run the database upgrade. Customer orders etc are all as they should be but I would like to be able to access the old admin pages so that I get things like the settings for the payment gateway and so on

Any suggestions? Is it possible that this is a .htaccess corruption rather than anything to do with the php?

Thanks