It was not hacked, nor was this a virus. You erased the hard drive, so there's no system for it to start up from. I'm not sure what "it won't let me install" means, since you haven't given a specific error message, detailed description of the problem or information about how you were trying to install.
First, before you do anything else, was the data on that drive backed up? If not, it's currently gone. You may be able to get most of it back using a data recovery tool. If you need to do this, do not do anything else with that drive, and see Recovering deleted files.
If you already had Mavericks installed before erasing the hard drive, hold down command-R at startup. This should start you up in recovery mode. From there, you can reinstall Mavericks. If you did not have Mavericks installed, or if you did but cannot get it to start in recovery mode, then you will need to reinstall an older system. If the machine originally came with Snow Leopard (Mac OS X 10.6) or older, you will need the install disks to do this. If it came with a more recent system, try holding down command-option-R at startup to boot into internet recovery mode. You will need to be on a decently-fast network.