hi i am new to this blog.
i have Western digital 80gb hard disk.
but now it is not detecting and showing 4gb in bios only
how to solve this . can any one help
how much it cost to repair this

First of all, you need a proper diagnosis.
You should find a DR company near you (if exists) or else outsource it another place, so a pro can check what's wrong with your drive.
Most companies here don't charge for diagnosis, so you just have to pay for shipping.