Tascam 414mkII 4 track playback problem

I've had this 4 track for about a year, and suddenly last night track one ceased playing- at first I thought it just wasn't recording, but I get an input signal fine, it just gives a rumbling sound when the tape source is selected, and won't play back previously recorded tracks either. Could this be as simple as demagnetizing the heads; would that cause such a sudden disappearance on just one track? Or is it more likely a mechanical problem, with the head itself or even the input selection switch? I did the easy thing and cleaned the heads, to no avail. Any advice is greatly appreciated before I start poking around in there.