Some apps just dissapeared

[mahesh]

First of all I want to say that I am new to Ubuntu, but I am not new to bash.
I am a software engineering student, and I want to learn to use Linux since I heard is much easier to code in.
I installed a few IDEs and some languages (Java, JS and .NET).
Everything fine, until later today I opened up my PC, and my drive wasn't mounting (a drive that I used as storage in Windows and is NTFS).
I installed ntfs-3g again and carried on with my work. Later today I randomly wanted to change the background and my settings app disappeared.
I want to know what causes those apps to just disappear, I don't know if other apps disappeared as well...
Thanks for reading this far. Have a good day