My iPhone 4 keeps freezing and force closing apps?

Recently whenever I open an app, it somehow always freezes then closes.

For example, when i'm using Skype, when someone is trying to send me a few messages at a time the screen would turn black and go to my home screen.

For Youtube, whenever I'm in the middle of watching a video, it would suddenly turn black and return to the home screen.

Another example would be when i'm using Messenger and i'm trying to write a message, but the screen ends up freezing and again, the black screen then home.

I don't know what the cause of this is, and how I can fix this?
I've kinda dropped my phone fairly badly 2 months ago, and I think this may play a big role in this freezing dilemma? -.-

Restart your iPhone, delete some of your apps if you have too many as well as your pictures.