Home page is not displaying

hello, I have uninstalled the K2 component, and unpublished unnecessary modules, also I have modified the css files of template for some color replacements. the website was working very well but suddenly I have started to have an error page instead of normal homepage of template. I have double checked every module, every menu item and every article but still I'm having problem. why is this happening? here is an order of what I have done...
1: I have modified the css files for some color replacements, and modified logo, and other photos for color replacement. The website was working well.
2. I have deleted all K2 items and uninstalled the component. there was an error page but i modified the modules connected to k2 and the site started to work well.
3. I have deleted unnecessary menus, and modified the main menu, deleted articles and categories and created new articles and categories. I assigned the modules to categories, double checked everything but still I'm having problem with home page, there is nothing but an error page.

please help me.

PS: The website link CLICK HERE

Hi,
it looks that one of your GK NSP4 or GK Image_Show module you non existing content as a source. Or you use k2 article in slide but you didn't add this article.

the problem is almost solved, I have unpublished the tabs module and the site is working very well. but I have another problem, when I try to display the register module together with login module, the website is gone. when I disable the register module, everything works perfect. how can I solve this problem?? I need the register module.

Can you give me what error do you have ? Or you don't have any one ?