On a sunny day (Wed, 23 Jan 2019 09:01:58 +0000) it happened Chris Green
 wrote in :

>Big Bad Bob  wrote:
>> On 01/20/19 01:33, Chris Green wrote:
>> > My RaspberryPi has rebooted itself a couple of times recently and
>> > ended up in a non-working state.  In particular it has no ethernet
>> > connection which is rather unfortunate as it's headless.
>>
>> > Power down/power up fixes it but having run for months without
>> > problems previously it would seem there is something wrong.
>>
>> overheating?  have you tried a CPU heat sink?
>>
>That seems unlikely given that it's winter now and it's definitely
>colder in the room where it is.
>
>
>> might also be something simple, like power spikes.  a Big Fat Capacitor
>> across the 5V input (with a 0.47uF ceramic in parallel with it) might
>> help.  Rapid power sags, caused by lots of possible things, is a likely
>> cause for unplanned reboots.
>>
>Possible I suppose, I guess I could simply try a different supply to
>run it from, I have quite a few different ones which I can try.
>
>
>> > Does anyone have any idea what might be going wrong?
>> >
>>
>> see above
>>
>Thanks for the ideas.

I think a bad wallwart is sure a possibility, the electrolytic capacitors in
those often go bad.
OTOH I have a Pi like yours now running about 12 hours a day since 2013 on the
same old wallwart:

http://panteltje.com/panteltje/raspberry_pi_FDS132_matrix_display_driver/index.
html
stopped every evening with 'poweroff' and has not failed to boot once, same old
card...
Sometimes I accidently unplug it.. it still reboots OK every time.
But I NEVER upgrade things that work (never repair something that works!!!)
Fixed IP address (could that explain something?).
One thing: your card does not happen to be full? I had that once and it caused
all sort of strange problems.
The pi above also gets its time from the LAN via the internet when it starts
up.

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