Im assuming this is a 96 onwards Ford Fiasco, if not then ignore this post.
Look in the fuse box in the passenger side rear of the engine bay, you will find two rows of small fuses. The row facing you as you look from the front of the car, 5th along from the left is the immobiliser fuse. Check this hasnt blown, and make sure that there are no empty fuse sockets there too.
I tend to find that people will remove the fuses to check if something else doesnt work, then replace this fuse in the wrong socket as there are usually 2 empty sockets there.
If its a pre 96 Fiasco, then make sure you have 3 black wires going to the negative post on the battery as most people tend to leave off one of the smaller earth leads when removing or charging the battery.
Hope this helps