XM6 Alarm Panel Not Responding?

My Alarm system based aruond an Optima XM6 panel has failed. I noticed the other day that the outside siren was sounding very quietly throughout the day. Looking at the panel, it just showed the Power LED but would not react to any key presses. I removed the outside sounder battery which was virtually flat when tested. I then removed the panel battery to find that, too, was pretty flat. Both were about 9yrs old. I replaced both batteries and everything worked OK until the next day, when the panel locked up again for no apparent reason. Any ideas as to what might be causing this? Any suggestions much appreciated!

many thanks.