|Right mine started to do this at about 15 months old, it had not been used for a few months.
So took it apart, to find the microcard inside as I assumed it may be the problem.
My first thought was why is it moving about so "moved" it about a bit and switched unit on, straight away it worked again, so assume that vibration could cause the card to disconnect, in my case to cure this I dripped some candle wax onto the chip and main pcb, still working at moment. Bluetack or similar could also work.
I'll try it out and see if it remains ok.
hope this helps others.