Zoom H4n problem

Sorry this is only marginally photography-related, but some of us use this device for improved audio when doing digital video. 

I've had this unit for a few years and it has worked fine, until today.  Now, when I attempt to record (using the integrated mics, haven't been using any other input source), my input levels are nonexistent, i.e., one bar shows at -48.  It does record the silence quite well, writes and plays back the files, etc.  I haven't changed any settings, nor do I know of any setting that would result in this.  Up until today (including yesterday) it was working fine.  The battery level did show as quite low so I put in fresh, but this made no difference.  It hasn't suffered any trauma, in fact is in what looks like mint condition. 

I'm baffled.  Any ideas? 

thx