Does your web app runs correctly from your visual studio (locally)?
There might be couple of reasons like
1. There might be some missing things (like end tag etc.) in web.config!
2. Your web application might configured with incorrect ASP.NET versions!
3. Application Pool settings might not correct!
If you are manually copy paste your web application, then you need to make sure only changing content should paste and not the whole things again and again! (like if you change content of one aspx file(not cobehind), then you copy paste only aspx; if you change codebehind, then copy respected DLL as well)
Alterwate way is to publish directly from VS! Please check link for that: