My window won't resize to take half screen when slinding it on a side

Hello ! I'm having a problem on my Ubuntu laptop. One day I woke up and tried to make my browser window (or whatever window) take half screen on the left by sliding it against the left border but it doesn't work anymore. Neither does it on the right or on the top (when I did it on the top I guess it maximized). I don't know why.

Could someone help ? By the way, english is not my native language and this problem was kinda hard to describe for me that's why I couldn't find anything on the web. Please forgive my bad english. I can try to explain again if it is very unclear. Thanks a lot ! :)