reinstall the drive. next you need to use your operating system reinstall disks or usb to reinstall the os. if you didnt create these when you got the machine, you will need to contact acer and pay for them.
once you have the system up, use wd data lifeguard diagnostics to check if your hard drive is functional. if it fails the drive needs to be replaced and you will have to reinstall the operating system on the new drive. you can get the diag here: http://download.wdc.com/windlg/WinDlg_v1_29.zip
this is the simplest, cheapest way to fix your issue and avoid bringing it to a tech repair place. if you have problems with any of this you may just want to take it somewhere. i repair machines for a living and most people aren't savvy enough to do this themselves.