CANON POWERSHOT A70

Your Canon A70 is one of those camers made with bad CCDs. That is the image sensor has known defect after production and market launch. Instead of recall their products, Canon offer owners to send/bring in their camera for CCD replacement without service/parts charge. Go to Canon web site to find an authorized service centre nearest to where you live, phone them up to confirm free service. You may either mail in or bring it to them in person. As far as I know, Canon will reimburse the shipping cost to you as well as sending the fixed camera for free.
Take a look of a third party comment/information regarding the bad CCDs by clicking here before taking further action. It didn't show black/blank screen as a symptom, but individual manufacturer web site did mention them as symptoms.