This can be one of two problems.
One of the elements in the oven has gone out. You will need to confirm that the elements are functioning in your unit. On bake, the bottom element should get red hot off and on during use and the upper element(broil) will get hot to help circulate the air.
they both are working bottom and top . If I put it on broil the top gets red .hot . If I try to cook with it at 250 degrees it still burns the outside of stuff.