Try these fixes if, for example, your phone randomly reboots, freezes or does not switch on, if an app crashes, if you have sound problems, or if the screen is blank:
1. Charge your phone
Charge your phone for at least 20 minutes.
2. Restart your phone
Press and hold the Power and Volume down keys simultaneously for 10-15 seconds. Release and the phone will restart. After this operation, you may need to set the date and time on the phone.
If the problem persists, reset and restore your phone as instructed below.
3. Reset your phone
This erases all personal content on your phone and restores the factory settings.
Before continue with the reset, make sure your phone’s battery level is above 50%. To check the remaining battery life, go to Settings > System > Battery saver (Lumia with Windows Phone 8: Settings > battery saver). If you are not able to check it, or the level is too low, charge your phone for at least 20 minutes.
Reset your phone by using the phone settings. Select Settings > System > About > Reset your phone (Lumia with Windows Phone 8: Settings > About > Reset your phone).
However, if you cannot access the phone settings, follow the steps below:
Disconnect the charger. Press and hold the Power and Volume down keys simultaneously. Release the keys when the phone vibrates and then immediately press and hold the volume down key until the phone displays an exclamation mark (!) on the screen. Release the volume down key.
Press the volume and power keys in the following order: volume up, volume down, power, and volume down.
Lumia 950 XL users: the physical key combinations differ slightly compared to other Lumias.
Wait for your phone to reset. During the reset you will see gears rotating on the screen for up to 5 minutes, after which the screen goes blank for ~30 seconds and then your phone will restart.
Go through the initial phone setup. Make sure that you have the date and time set correctly. Select Settings > Time & language > Date & time (Lumia with Windows Phone 8: Settings > date+time).