First, please note that myASP.NET systems is fully secured by professionals. However, there are ways for hacker to hack your websites. Most common ways is by finding loopholes in your applications.
You should examine your application thoroughly and see where the problem might be. But this process is extremely difficult.
In the meanwhile, we suggest you to also lock down your file permission by using our File Manager-> File Permission. Set folders that doesn't need write permission to READ ONLY. This will reduce your chance of getting attacked/hacked.