Sounds like you've done something you shouldn't have. Or maybe it's just how you worded the question. Did you maybe replace the hard drive?
Anyway, whatever the cause you have a problem! Thankfully all recent Macs have a recovery partition that will allow you to re-install OS X or restore it from a backup.
Start up the Mac and after the "bong" tone, press & hold down the Apple key and the "R" key at the same time to start the recovery partition. This takes a few minutes to start so be patient.
If you don't have the recovery partition, the Mac will attempt to use the Internet Recovery function so make sure you can connect to the internet.