If your iMac is not seeing the system folder when booting up there is two things you can do.
Once booted up, in OS X System Prefernces Pane in the OS X Dock, look for and click on the Startup disk icon.
Your main OS X system should show up in a horizontal window. Clicl this System disc icon once, than at the bottom of the window click on the lock symbol.
Either restart or completely shutdown your iMac as see if it boots to your hard drive much quicker.
If this still doesn't work. hold down the iMac's ppower button for 10 seconds to force power down your iMac.
Wait another 10 seconds then press the power button as normal and immediately hold down the Option key on your keyboard to try and invoke the OS X Startup manager. If your hard drive appears on screen, pick this drive for startup.