Find your nearest acer service center and ask for help. Maybe it's not because of the harddisk. Just make sure of it first.
Assume it's because of the harddisk, if you want to save your data, you might want to find a service center who can deal with failed harddisk. If not, you can just buy a new notebook harddisk and replace the old.
You can buy notebook harddisk at most computer stores. The cost depends on the capacity and type. Just ask the computer store.