When you create a schedule using AlarmManager and you found that it stops working after phone reboot. So to avoid this problem we need to add Broadcast receiver that should receive "BOOT_COMPLETED" broadcast.

For this you need to register your BroadcastReceiver in android manifest file

<receiver android:name=".BootReceiver">
      <intent-filter>
           <action android:name="android.intent.action.BOOT_COMPLETED" />
           <category android:name="android.intent.category.HOME" />
       </intent-filter>
</receiver>

Here BootReceiver is your BroadcastReceiver

Now add the following permission in manifest:-

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>

this permission is required to listen/reverie the BOOT_COMPLETED action.

Create a BootReceiver like this:-

 public class BootReceiver extends BroadcastReceiver
    {    
       public void onReceive(Context context, Intent intent)
       {
          //  execute when Boot Completed
          /**Schedule your Alarm Here**/
         Toast.makeText(context, "Booting Completed", Toast.LENGTH_LONG).show();    
       }    
  }

When boot completes, the onReceive method of BootReceiver wil execute. Here you can schedule your Alarm again or start your service again.
Thanks