In the display screen of my Tappan gas stove, a symbol (looks like half of a capital "H" followed by a dot) appeared and I can't turn on the oven/broiler, or clear it (none of the buttons work). However, the burners on the stove top work. What does this mean? How do I fix it so I can use the oven/broiler again? Please help, I'm desperate!

Kitchen Ranges - Tappan - 24 in. Freestanding Gas Range Kitchen Range