On e reason this may happen is the hard drive has an error at critical point in the boot up. Maybe try another hard drive or use a usb bootable linux drive if you do this and it starts it is likely to the hard drive. You need to replace it.
Once you have working drive you will need to reinstall the system etc. If you buy a similar sized hard drive you may be able to clone the recovery partition and thereby restore your system or you have made recovery disks you may be able to restore the computer quite easily and then try to take the data off the old drive