I can feel a hard lump on the roof of my mouth.

If the lump is in the middle of the palate, feels hard, and has been there for a long time, it is most likely a palatal torus - a thickened growth of bone. A torus is a benign bony protrusion, not a tumor or cancer; it is painless, grows slowly, and generally needs no treatment. Removal is considered only when 1) it interferes with making a denture, 2) the surface is frequently rubbed raw and injured, or 3) it significantly affects speech or eating. However, if it appeared suddenly and recently, is growing quickly, has a surface that has stayed raw for more than 2 weeks, or feels soft, it may not be a torus, so please come in to have it checked.