Trying to put enum type in seperate .h file, compiler say it's not defined

Question

I wrote the header.h and header.c

The function get_key() is returning the key that has been pressed, worked fine when i put the enum and the function decleration/implemention all in the main() file, but when i tried to seperate to two files, here's what happened.

header.h

    enum key
    {
        UP_KEY,
        DOWN_KEY,
        LEFT_KEY,
        RIGHT_KEY,
        INVALID,
        ENTER,
        NONE


    };

    key get_key();

HEADER.c

#include "header.h"

key get_key()
{
    int ch;
    if (kbhit())
    {
        ch = getch();
        ch == 0 || ch == 224;
        switch (ch)
        {
        case 0:
        case 224:
            switch (getch())
            {

            case 72:
                return UP;
            case 80:
                return DOWN;
            case 75:
                return LEFT;
            case 77:
                return RIGHT;
            default:
                return INVALID;
            }
        case 56:
            return UP;
        case 50:
            return DOWN;
        case 52:
            return LEFT;
        case 54:
            return RIGHT;
        case 13:
            return ENTER;
        default:
            return INVALID;
        }
    }
    return NONE;

}

Two problems.

1. In the header.h, at the function declaration, the compiler says that key is not defined, which is strange considering the key is declared as enum just 2 lines above.
2. In the .c file, enum key is also not defined, which is also strange as I wrote #include <header.h>.

What could be the solution?