Rotation Locked

On my iPad 2, Settings/Genral/Lock Rotation, is unchecked but the screen is locked in vertical position. I have held the sleep/home button down for several secnds and have re-strated the iPad to try to unlock it. A "Lock Icon" had appeared at times in the top middle position above the screen but is no longer displayed. A lock icon with a circle arrow around it appears at the top right side above the screen.

How can I unlock the rotation so that I can use it the iPad in both the vertical and the horizontal positions?

It had been possible yeseterday but was locked when I woke the iPad this morning.

I would appreciate your help and suggestions.

Vernon