I have the same phone, but only occasionally have this type of problem, but in my case it's usually because some stock apps/widgets(like battery save mode, nighttime/quiet time mode settings etc) can turn off some features of the phone when certain conditions are met, like it being a certain time of day/night/day of week or the battery being lower than a certain %, can shut off among other things, your data, which is gonna cause you to be unable to make calls, get/send texts, youll just have zero bars. And i've noticed this phone doesn't always set the phone back to the way you had it before the conditions were met to change the settings. Easy fix for me though, i just have on the pulldown screen a kind of widget that gives me abunch of buttons that let me turn on/off things like data, wifi, sync, airplane mode and so on. (edit- Or, you could change the settings for the battery save mode or whatever it ends up being to NOT turn off your data, or disable that app/service altogether and just figure out another way to conserve battery life when it gets low)
Don't know if any of that helps you, but in any instance IME with this phone that i couldn't send/recieve anything was because for one reason or another the data/wifi setting were turned off. Try just giving all your settings/configurables a good once-over, see if maybe something's turning it off.
