modification od the php file in wordpress

Now,I am making website. I customized template php file. Then each category top page can not display thumbnail image in each catogory top page dispite it can display before. The work that I customized is as follows. 1.I added custom field each post page. 2.I customized [login to view URL] that to add external link if I input value in the customfield. In the above, I think that the cause of this ploblem is content in [login to view URL] and [login to view URL] file. I want to be able to display thumbnails image on toppage in each category page. Is there the engineer who can solve this problem?