There should be a metal hinged door that you have to open to access the cash can. That door usually has a switch connected to it that detects if the door is either open or closed.The type of switch depends on the brand of bill acceptor that your machine has in it. I'm willing to bet this is your problem.
Also, there should be a white button on the very back center of the housing that the cash can slides into. This is the Cherry switch that senses if the can is inserted all of the way. Check to make sure the wires are connected to the back of this switch and that the switch and that it is operating properly. You'll be able to see the white plunger for this switch by pulling out the cash can and looking at the inside, back of the chamber that the can slides into. Is your bilval head black, yellow or blue? Post some pics so we know what you're working with.