Permalink problem

Hay I am in great problem I use word press for my website. I have already posted more than 6oo topics in my site. I used to set my permalink category +post name but unfortunately today I changed the custom permalink and it appears totally nothing found in the pervious post link. and I could not solve the problem. now my site does not show any post? what can I do please help me.

http://wordpress.org/extend/plugins/post/