1
I was ill and I lost a lot of lessons.
I was ill and I missed a lot of lessons.
If you do not go to a class, meeting, event, etc., use miss, not lose: ‘I missed two lessons.’ ‘We missed the beginning of the film.’
2
We were late and lost the train.
We were late and missed the train.
If you arrive too late to catch a train, bus, plane, etc., use miss, not lose: ‘Hurry up or we’ll miss the plane!’
See LOOSE