IPhone 4 screen doesn't appear most of the time or glitches?

Okay,
So…
The screen itself isn't damaged except a small crack, and the phone itself still works fine. It's just that most of the time the screen won't show anything but either darkness or whiteness, or if it rarely shows anything then it's constantly glitching out until it stops working.
I know it's not a software problem because I have tried resetting the phone via the phone's setting and also tried using iTunes to completely wipe it and re-install everything and it's fully updated.
When the screen isn't showing anything, the touch recognition still works. (I can hear the bleeps and when the screen eventually works again it's opened to what I tried opening or such)

Does anyone know what's wrong with my phone? And if so, do you have any idea of what it'd cost to be repaired professionally?

Added (1). *a small scratch, not crack.-.

I think the wires that connects to the screen and the office are loose. This is happening to my friends phone. You might have to her the screen replaced or possibly just the wires put properly

I think your wire connection is loose.

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