what do i do if my iphone doesn't show up in my itunes

I called Apple a few hours ago, because I was unable to sync pictures from my phone to my PC. After talking to two technicians and uninstalling and re-installing iTunes, my phone finally showed up in the My Computer folder. I was able to sync the photos to my computer. When I was ready to sync other pictures to my phone, I switched to iTunes. To my horror and dismay, my phone was nowhere to be seen in the device list. I still can't figure out how to get it back. Someone please help!!