You can ground the sending unit and see if the gauge moves. If not, the gauge might be bad (assuming the key is on and the signal wire has continuity).
Otherwise, the sender could be bad or the sending unit wire has an open.
is it wired correctly...could the sender be bad? WHen the car warms up...what kind of resistance do yuo get from the sender? Have you done anything recently that could have damaged the gauge wires, etc. You gotta be more specific?