Generally this problem can become from the 2 following problems are:
1. The device have the Stuck or Brick problem and for result device will be lock in fastboot mode.
2. The user enter the fastboot mode but don't know how to get out from that.
But the issue becomes a little complicated to resolve when the device have a feature like not removable battery by default, for to the device can't force power off with remove that battery, for resolving that problem actually is easy way, you only need to press or hold the power button in one minute, if its success the device will force shut down in automatically.
Thus, the problem stuck in fastboot mode resolved, for next after the device force shut down or in condition power off, the next step is for fixing your smartphone from the brick or stuck issues, for that you can using the fastboot method or flashing the new ROM or firmware, after that your device will work and boot in normally again.