Heads in my HTS541080G9SA00 are going (maximally) only to half of platters. Whats the problem ?

I have two questions in return for you :

1) Why did you open it ?
2) How much do you know about HDDs in general and HITACHI in particular ?

The heads are trying to read the service area of the disk. This is where critical drive-specific information is located on the drive surface. If it can't read that area, the data will be unreadable. This is a task for a professional because the heads will have to be replaced. Hopefully you didn't touch anything inside when you removed the cover; this will only complicate matters.