sinumerik 840D

I Have a turning machine with sinumerik 840D control .
the machine's software (NC,plc)is down with massage plc is stopped appears .
i download and restore all plc nc files correctly and sucessfully .
my problem is "after hours the plc stopped and down"
also i download the plc again , and after short period plc files down and message "plc stopped appears "
so why the plc files are not stable and dropped after downloading it

Sorry, I saw post just today
Hope you found a solution why PLC-controller stopped. You need toopen PLC with Step7 to make diagnostic of fault.