|"..hard disc fell very gently.."|
"..some noise in the hard disc, not as smooth fan noise as earlier..."
If it had fallen as gently as you suggest, it would not be damaged and making more noise than it did before. Did you actually see it as it fell, including it hitting the floor?
What you are hearing is probably the cooling fan in the case, not the hard drive.
Even if the external case is not seriously damaged, the external drive cannot be recognized if there is anything wrong with the cords to it.
Make sure the plugs on both ends of the cords are plugged in all the way. Examine the connectors on the cords to see if they are damaged.
Examine the plug on the end of the power adapter cord to see if it's damaged.
If the external drive has one or more leds that light up, is it/are they lighting up as it/they did before?
If the drive uses a USB cable with a larger USB connector on one end and a smaller USB connector on the other, it's a standard cord and can be replaced easily. If the drive has a 3.5" hard drive inside, the USB cable may be the same as one used for a USB connected printer. Try another USB cord, or replacing the cord with a new one
"I have 80GB of data inside which is important. "
Most people think their data is important.
It's never wise to have data you consider valuable in only one place. At the very least, you probably have a burner drive - copy or backup the data you don't want to lose that you can't re-install to CDs or DVDs.
What's the capacity of the hard drive in it?
What model is it?
"Is there any chance of open it and repair?"
You haven't revealed you have any troubleshooting skills so far, and you haven't said that you have checked the cords I mentioned. If you have no expertise regarding repairing things, what would be the point of that? Most things that were assembled can be un-assembled. You can probably open it up and remove the hard drive, but other than that you may not be able to repair what is wrong. It would probably cost you less to replace the external enclosure - it can be any external enclosure that is compatible with the hard drive's size (2.5" or 3.5") and connection type (IDE or SATA ) - rather than having to pay someone to fix your present one, if it can be repaired.
See the last paragraph of response one - it's very unlikely there's anything wrong with the hard drive inside the case itself.
Laptop users often use an external hard drive because they think the hard drive on the computer doesn't have enough capacity.
If you are connecting this to a laptop computer, you can easily and inexpensively install a hard drive in the laptop that has more capacity.