Is anyone else having this problem?

I have a login lightbox on my site which has a button that redirects to a signup lightbox if you do not yet have an account. It had been working fine for the past couple of months until a few days ago when I noticed that the button does not work. It redirects to a blank page. I have not changed the code at all, and the button isn’t even linked with code (I also tried to change to code but that didn’t help).
In preview mode it works fine, the problem is in the live site. This is a huge issue as no one can sign up now and I am losing potential members!
Has anyone had this problem or knows what to do??
Thanks