It’s a shame, but things happen. Log into your LiveXP profile, mark the lesson as "The lesson didn't happen" and leave a comment that you couldn't come to the lesson. In case the tutor agrees to reschedule, then choose "We agreed to reschedule" reason and indicate the new date and time so that the Support Team can move the lesson to a new date and time in the LiveXP calendar or send a reschedule request to your tutor if you want to reschedule the lesson which has already started. To send the reschedule request, follow the instructions described here.
Generally, try to let your tutor know about any issues in advance and send reschedule requests whenever you feel like you cannot make it to a lesson. You’ll have more time to agree on a new day and time for the lesson.
If you couldn’t reach out to your tutor in time, then just book a new lesson with them.