The connection to SQL server has been lost. Check your network settings, check the computer running SQL server is online and on the same network as SambaPOS. You have not given much context here however I assume that you had SambaPOS working already on your network.
This is unlikely related to ending work period, however as I would guess this is being done at the end of the day, possibly someone has turned off the computer where SQL server is installed. All terminals must have access to the computer running SQL server to function, this is by design and not any issue with SambaPOS.