It depends on what happened *after* you showed. IF you stayed on onsite, whether or not you could work productively, then you would have to be paid. If, on the other hand, you were sent home because the equipment wasn't working, then if you are an hourly employee (not salaried), you only have to be paid for the time you were actually "working"--i.e. the time after you showed, but before you were sent home. If you have some contract or agreement (including a union agreement) guaranteeing you a full shift pay for any day you show up, then that agreement would have to be honored and you'd be entitled to the pay.