cant log in Admin v1.5.1

Hi

I tried searching but cant seem to find a solution.

I cant log into my admin panel and my customers cant log in either?? No idea why all of a sudden is stopped working. When I enter my user name and PW it seems to refresh itself instead of logging me into my admin, been using v 1.5.1 over18 months and this has never happened before, always ran perfectly before

I was told to add a ocreset.php but that's done nothing to help.

Can anyone help please?

thanks