Membership plugin membership page broken

Hi there

The site that we are working on was working and ready to go live. I have just done one last test and it seems to have broken. This is strange as the site has been left for about 2 months and no changes have happened in this time (that i am aware of).

With the membership plugin when i try to choose a subscription to sign up to, when i click 'sign up' nothing happens - i stay on the same page with the subscription options and cannot go any further.

Can anyone please help?

Thanks in advance