It's likely the digitiser (Touch Screen portion of the screen) is damaged and needs replaced.
To be honest as this is quite an inexpensive phone it's probably a better option to sell it for spares and buy a replacement. A replacement digitiser and the time to repair wouldn't really be worth it.
But if you do want to repair it you would need to pick up a new LCD + Digitizer. They generally come as one piece.
It's possible to buy the digitizer seperately but it requires specialist equipment to remove the old one from the screen you have and is a complicated repair.
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My phone fell off the table and now the touch screen isn't working.
Can i fix it?
par Amar's problem