Dawn Redwood Bonsai Forest
You may have bought or had one of these given you in the past and have been a tad disappointed when it died - as I'm 99% sure it did. The reason it died is most likely that you kept it indoors, and they do not survive indoors for much more than a few days. This is essentially an outdoor plant.
Dawn Redwood Forest - NB: This photo is not our own
Another reason could be that they have a sleep time or down time as do a great many of our native trees in the UK. The down time for the Bonsai Dawn Redwood is pretty much the same as our own winter here, around the end of November for 3 months or so.
I shall start growing in early March 2016 and I would like to sell the resulting crop but I have severe doubts that something this fragile would survive normal postal delivery, or what passes for normal with those people.
Anyway, during the down time they need to be placed outside, preferably without their pot, and somewhere where they can catch the winter sun. Dig them up again in March/April and pop them on the patio to be admired by your visitors.