Password protected page not loading

Hi

For about 3 months I have had a password protected page on my website. Just lately when people try to view the page it does not log them in.

They are prompted to enter the password but once they press submit the page just refreshes.

I have tried all I can think of to rectify the problem. I have updated my WordPress installation manually, checked my table structures in MySQL, deactivated all of my plug-ins, tried changing the password, disabling the password and then turning it back on again and I have cleared my cache numerous times. I have tried using different browsers and versions but nothing seems to work so I would really appreciate any ideas.

Thanks