Normally (in vehicles) that means the radio does not have a constant 12 volt power supplied. Perhaps the fuse for the 12V constant has blown. Either a
in-line with the wire or located on the back of the unit.
Since the clock does not need to be reset could mean that the clock has it's own "Battery" internally and the presets require constant 12V power and it is not there to keep the presets set.
Please let us know once you have located the problem. I am curious as to what the problem really is.