My phone keeps rebooting

This summer holiday I haven’t even used my Alcatel, but all of a sudden it was kind of dead, as if battery was empty even when charged fully the day before. Battery condition was still okay when I left for the holiday…but i noticed that when battery charge percentage came close to 60%, the phone turned off. So, anyone else with the same problem? Could it really be a dead battery issue? Dumbest of all…did not backup recently, so all valuable info for me is still on the phone :-(