Where did they go?
I rebooted my mac and reinstalled the Macintosh HD but when it all started, I don't have iphoto and imovie anymore. How and why did it happen?
Do you mean you erased your Hard Disk and renstalled the Operating System?
If it is then iMovie and iPhoto are not part of the OS. Restore them from your Install Disks (if your Mac came with them) or the App Store (if it didn't)
Other than that, I'm not sure what you mean
I rebooted my mac
Means I turned my computer off and back on again
reinstalled the Macintosh HD
means I took the hard disk out and put it back in again
Sorry if I've misunderstood you.
Regards
TD