If you have installed an app capable of blocking your Android smartphone at startup, there is a possibility to remove it and to start Android in the Safe mode.
For the smartphone Samsung you have to turn on the phone and immediately press the key volume down until the system is loaded. You will notice that you are in Safe Mode by the writing that appears below. In this way it will be possible to delete the offending app.
For the smartphone LG, Nexus, HTC e Asus you must, with the phone switched on, press and hold the power button until a new screen appears. Here you tap on switch off and hold down until a confirmation message appears. Choose whether to restart with Ok o Undo.