Sub Domain adding index.php

Hi,

I have a website which runs in Zencart I am in the progress of moving this over to Opencart. I have Opencart installed into to a sub domain /Store/ . It's been working fine and I've had no problems with it until, I have two weeks off and neither website has been touched in this time. Now when I go to /store/ it take me to a Zencart 404 error page. If I enter /store/index.php it takes me to the Opencart site fine. I never had to do this in the past and was wounding if there was away around this.

Thanks
Kenny