Can't make changes to my site…

I am having massive problems.

I can’t make changes to my site without it corrupting all the links.

I can add product, but that’s about it.

At the moment I need to add a plug-in that will enable me to take details for a mailing list, so I have selected ‘EASY SIGN UP.’ Installation and activation is OK, but if I try and add a SIGN UP page with the code included, it corrupts the site. All the category links and product links come with a 404 error.

Please can someone out there help.

Thank you

Jason