Sometime, in the middle of adjusting a project, the height of all tracks happens to be minimized. I don't Know why it happens. Maybe I clicked something wrong. I Know the way to adjust this issue, but I'm tired that this sometime  happens.

Maybe accidentally enabling Auto Track Zoom with SHIFT+Z or
 

Maybe you're hitting ( A) Show All tracks or (F) fit all tracks.
 
You can also zoom with the keyboard.
 Key
What it does
CTRL+UP ARROW
Zoom out vertically
CTRL+DOWN ARROW
Zoom in vertically
CTRL+RIGHT ARROW
Zoom in horizontally
CTRL+LEFT ARROW
Zoom out horizontally
Hold down Z
Arm the Zoom tool
ALT+Z
Undo the current zoom
ALT+SHIFT+Z
Redo the previous zoom
F
Fit tracks to window
A
Show all tracks
SHIFT+F
Fit project to window
SHIFT+double-click a clip
Maximize track height
F11
Full Screen mode