To lock your bootloader you’ll need to pass some Steps like when you unlocked it.
This will not restore the factory default lock, but will simply re-lock the bootloader so no further changes can be made.
2. Run Command window
Open the folder, press and hold SHIFT key, RIGHT-CLICK on your mouse and select Open command Window here
3. Boot in Bootloader mode
Remove and reinsert the battery (if the battery on your device is not removable, long press the power key then select restart, now hold down the volume key while restarting to boot in Bootloader mode), press Volume Down and Power to start the device into Bootloader mode.
4. Plugin device and Lock
Plugin your device into the computer using a USB cable.
In the command prompt type:
fastboot oem lock
After short time device will reboot saying your bootloader is Locked.