This video will teach you how to reconstitute your very good markers. When I taught children drawing using the Monart method I used markers like these. They are serious markers and it is only worth it learning how to wake them from the dead because they are quite expensive. (It’s not for the inexpensive crayola markers)
If you do however, use prismacolor markers then you may want to watch this video which will tell you how to get them working again.