Hard disk error -

hello sir.

I am lokesh. I am new to this forum

I had one 500GB seagate hard disk.
its internal had disk of my system. there is one problem with it.
my system says. "disk error" or "hard disk is missing" or "disk failure". - like these.

I don't know what happen..
someone please guide me.or help me
what to be done? what is the problem?

how to fix this problem?