If you go to the Archive page, you can "stash" the database for later viewing. The microsd card can hold hundreds of them, although I've never tested the performance with more than a dozen or so.
If you don't use the archive, then HeaterMeter will try to restore the current database from the backup which can hold up to 24 hours of data. However, this relies on the time on the device being accurate when the check is done. If it has an accurate clock (which requires network access at boot time) then it should still be able to use the backup. I'm not sure if the time zone setting can mess with this, but the HeaterMeter should always be set to UTC time zone, since the times are adjusted in the browser. Also be sure your scale on the graph is set to Auto or 24 hours to be able to see it all.
i did an archive at 11:53PM, but there is no data in it. The csv has only a handful of lines with none of the temps. The unit was off from 4:30pm until just before the archive. It must not of restored the DB at the time I did the archive.
Yeah if it has been more than 24 hours and it wasn't stashed, then it is for sure gone. It should have continued the old database when you plugged it back in though (when you checked it at 11:30pm). You might want to try sitting the HeaterMeter on the table with a probe plugged in for a few hours and then pull the power and plug it back in to make sure it continues from about where it left off just to be sure the feature is working properly for the future though. It can lose up to about 5 minutes of data since the database isn't continuously backed up during operation, to save writes to the microsd.
The archive is just an extra copy of the current database, so if there's no graph data on the home page then it won't be in the archive either, just for further information.