On the left and up side, you have a button "Y="
Then enter your function...
For graphing, "Graph"
Can you tell me what's in "window" ?
And are you sure you're in the right mode for graphing ? (there is parametric, polar, etc...)
Have you ever saved x as a constant ? I mean sometimes it occurs that in programs you need to give a value to x, and that it has kept it in memory...