If the glass cover is circular it will need to be unscrewed anticlockwise - a damp cloth may help you get a better grip - burnt fat may have gummed it up a bit.
If rectangular then a large screwdriver will prise it away from the clips that hold it.
There's always a possibility that it's circular but needs to be prised out but it would be unusual.
Good luck...
Peccavi, July 2009
