everything in playlist disappeared after I open the program the next day
I can not tell you why. Maybe it was a user mistake by accidentally choosing Remove All from the Playlist menu. I have never seen it magically disappear. As you can see, the Timeline does not depend on things being in the Playlist; they are independent. You can simply re-add the missing items. You can even add them from the timeline by copying a timeline clip and then clicking ‘+’ under the playlist.
Ok thanks for the advice