How old is the emonSD install/image Dave? Assuming this is an emonSD when did you last update via emoncms admin page? If you have updated, did you update the rfm2pi FW too?
This concerns me as all the later images have log2ram installed and even if you did an ungraceful power cycle (pull the power plug) there should be old logs in ram. So is this an old pre-log2ram image or is there issues with log2ram I wonder?
We really need to look at the emonhub.log prior to and around the time of failure, even an uneventful log tells us something. If it happens again please copy the emonhub logs to disk before rebooting if you suspect log2ram isn’t installed or functioning.
The current log will be /var/log/emonhub/emonhub.log, In the case of log2ram, this file is rotated and saved to the /var/log.old/emonhub/ folder each hour so it may appear to be only “since boot” if it’s rotated since reboot. When you gracefully reboot the logs are saved to /var/log.bak during shutdown and reloaded to ram on startup, so there should always be some logs to be had, whether they go back long enough for the time in question depends on how soon you check and how fast the log messages are flowing, but for the timescale you mention, there really should be relevant logs if l2r is working as several logs are retained, possibly zipped.
The most common cause for rfm modules to stop receiving has been low voltage, the rfm modules are more sensitive than the Pi’s and can brownout without the Pi being affected if voltage is just a tad low. Have you changed PSU or added any ancillaries to this Pi recently (temp sensors pulse counters etc).
Another test you can try before rebooting/restarting if it happens again is to reset the rfm via ssh see
for a simple commandline utility to reset the rfm2pi.