I'm assuming that all the other functions work normally...the boom, dipper and bucket...also the loader? Is this issue happening on both sides or just one? Assuming that the hydraulic pump didn't just go overnight, which CAN happen, this sounds like it could be a check valve problem. I'm not familiar with a 480C at all. I have a 580B. I'm currently diagnosing a similar situation on my backhoe. None of my backhoe functions work. Machine has been sitting for about 3 years and was working fine when parked. I rebuilt the swing cylinders 3 years ago and just put them back on a month ago, only to find that the backhoe doesn't work.
Sorry if I'm being longwinded, just trying to explain the process I went through and what I've found. I've been troubleshooting for a month and just today remove my main valve block. When I took out the check valve from one of the sections, it was missing an O-ring, both on the upper and lower.
I know your stabilizer control valve isn't the same as the backhoe valve body but it DOES check valves in it. I have yet to do anything with mine as I'm rebuilding the whole valve bank and I just got it out and disassembled today. I would imaging the lack of O-rings on a check valve might produce symptoms similar to what you describe. Just a thought...