You either used Boot camp to create a Windows partition or you have, had, Windows installed and now it can no longer boot to Windows.
At startup hold down the Option/alt key to get the starup Manager screen. When that comes up if there is a Macintosh HD entry, and or one that has OS X installed on it, select that. If the system boots to OS X your good to go. Then remove the Boot camp partition by running the Boot Camp Assistant app again.
If the system fails to boot to OS X or there is no listing for OS X then more than likely you have a hard drive problem.