Terrain missing after downloading Classic Pack and Minecraft Music

Initially I was was having trouble with the music when I got thr jukebox, I went on youtube and found out that I could download music in minecraft store, and I browse and found the said item and download it. 

 After download the music, I continue going through store and found the classic pack, so i downloaded it.

 

So while I was playing I found out the terrain went missing as shown in the attachment. Please assist thanks.