Could be the starter motor but more than likely it's the battery that's dead, try another battery or jump leads first (make sure you connect them correctly!)
I've got an Escort which recently did the same thing and it turned out to be the battery. I even struggled to jump start it. The voltage would drop from 12V to about 6V when i turned the ignition on.
Just another possibility, is there an immobiliser fitted and does it work off of a key fob? or a special key? If so try the spare it might be that that's broken
Could be the starter solenoid or that the starter motor is stuck in mesh. Put the car in gear and rock it forward and back 9with the hand brake off) that normally would release a sticking starter. If not the above you have a dicky battery.