I have been design an asp.net site with .net framework 3.5 all thing is good until my site hacked suddenly it says a chines group and some other text by picture jpg they put files into the ftp server I delete them and they put them again I change all password and they do it again I think they are some script programmer and I can do nothing by deleting method, please help me.
I think it's pretty clear that you don't have the technical skills to be able to complete this job -- it's a difficult problem, with many potential areas of exploration and avenues of attack that need to be secured. I would recommend engaging the services of an experienced systems administrator to investigate these options, cleanup the infection, and deploy the appropriate safeguards to prevent it happening again.
As a developer, you should know if your app is secure or not. In the other hand, even if you have the most perfect and protected app, if there is a huge hole at system level, your programming skills won't worth too much and you'll be cracked anyway. Just hire a sysadmin so your system can be properly secured.
Yes womble I know I am developer not system administrator but I don't know where is the hole they attack again and again I think it is more complicated that you can explain but I want to know about how to prevent I know how to pay for system administrator and how to find him I want to know how they attack I had upload by some script on the ftp server by Injection or not some else method ?