Please answer compumac's question: "backup partition. Does Windows Explorer see it?"
We need to find out if it is Acronis playing up or if the Partition is. Maybe it has lost it's Drive Letter?
You said you have been storing all your backup images to an external drive. Is this problem partition on an External usb Harddrive?
If so, At what point did you connect the usb ext harddrive? Acronis has to have any ext harddrives up and running before you start the program (also before you use the Acronis Boot CD) or it can't see them.