My T720 has been acting up for the last year. I found it only does so when I leave it on non-stop for over 2 weeks. After those 2 weeks, it will start doing quirky things like not charging, freezing, cover light not turning off when closed (what a pita when sleeping), empty phone book, not ringing even though ringer is on, alarm clock not going off.....
After a simple reboot (turn off and back on) all is good for another 2 weeks.
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