Why won't my Pentax turn on with fresh batteries? It worked just a couple of days ago!!

Try cleaning the battery contacts with a pencil eraser. Even though the battery enclosure is closed off, there's still the liklihood that the contacts can get dirty and prevent complete flow of energy to the camera. Also, make sure those batteries are at least 2500 mah for the high power needs of your digital camera.

Also, were these batteries bought in bulk? Often, when you purchase batteries in bulk, it isn't uncommon that some or many of them are near dead or worse. Batteries bought in bulk tend to have a long turnover and such, can lose much of their battery power before they are used. And many batteries such as AAs are general purpose batteries and are not designed for the high energy use of digital cameras and the flashes they trigger. As such, I recommend using high energy batteries designed for digital photography. Varta is a good brand. They are usually sold in camera shops, are long lasting (because they are sold in small quantities) put out alot of energy.