blocked cellphone. Booting only with charger

hello guys,
sometimes my atrix gets stuck and when i keep pressed power-button + volumeDown-button it freezes", i mena it doesn't reboot, that happens only with the help of a charger. What could it be? Maybe's a battery issue? Once it was at about 30% and then it turned off and i had to spend a lot of time waiting until it awaken. Thanks in advance