Can't access my wordpress backend

Hello,

I recently downloaded a complete security plug in for my website as it was showing as not having a firewall.. As part of the security measures I was asked to set up a new link for logging in to the back end of my site.. all was fine until I actually tried to use the link, it brings up the log in page fine but once I log in it takes me too a page that states “oops, page not found”

I have tried using the old link to log in but that also states page not found..

I am completely shut out of the back end of my website and am beginning to panic!

PLEASE HELP!

Thanks
Sam