C Getting decimal input from user issue

To figure out why your code doesn't work, I suggest you post your full code, because problems may lie in the way you call this function.

So before full code is posted, I can just tell you that this code works well on my machine:

#include <stdio.h>
#include <ctype.h>

int getlong();

int main() {
    printf("\t%d\n", getlong());
    printf("\t%d\n", getlong());
    return 0;
}

int getlong() {
    long sum = 0;
    int character = fgetc(stdin);
    while (character != '\n') {
        if (isdigit(character)) {
            /* convert from ASCII */
            character -= '0';
            sum = sum * 10 + character;
            character = fgetc(stdin);
        }
        else {
            character = fgetc(stdin);
            continue;
        }
    }
    return sum;
}

ctype.h is included in order to use isdigit(), while tells you whether a character is decimal digit.

But in fact, you don't have to do everything on your own. Using standard library is more effective and efficient, both for you and for the computer.

For example, you can scan a long integer directly from stdin:

#include <stdio.h>

int main() {
    long value;
    puts("Please input numbers:");
    while (scanf(" %ld", &value) != 1) {
        puts("Only numbers are welcome:");
        scanf("%*[^\n]");
    }
    printf("%ld", value);
    return 0;
}

Notice the white-space at the beginning of format, this makes scanf() discard all white-space characters(including spaces, newline and tab characters) extracted until a non-white-space character is met.

Or, use strtol(), while is relatively rarely seen:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char buf[80];
    char *pEnd;
    long value;
    do
    {
        puts("Numbers please:");
        if (fgets(buf, 80, stdin) == NULL)
        {
            perror("fgets()");
            return 1;
        }
        value = strtol(buf, &pEnd, 10);
    }
    while (*pEnd != '\n');
    printf("%ld", value);
    return 0;
}

Of course, sscanf() also works, you can just write the code on your own.

From comments:
an extra newline in the stdin buffer...
Try replacing your current method with scanf() using following format string:

char* fmt = "%[^\n]%*c";

It reads everything up to the newline, then consumes the newline. * is an assignment suppressor.

Example: (includes functions to convert input string to float/integer number)

float get_float(void);
long get_int(void);
int main(void)
{

    float num_f = get_float();
    long num_i = get_int();

    return 0;
}

float get_float(void)
{
    char input[80];
    char **dummy={0};
    char* fmt = "%[^\n]%*c";
    printf("Enter floating point number and hit return:\n");
    scanf(fmt, input);
    return strtod(input, dummy);
}

long get_int(void)
{
    char input[80];
    char **dummy={0};
    char* fmt = "%[^\n]%*c";
    printf("Enter integer number and hit return:\n");
    scanf(fmt, input);
    return strtol(input, dummy, 10);
}

Note: These functions are bare bones illustrations of how converting input into number variables might be done, and were written without any error or range checking. As the commenter has stated, it would be worth your while before implementing production versions to read up on strtol() and strtof in detail. (Links are to the Linux man pages, but because both functions are part of the C standard libraries, documentation can also be found on MSDN here and here)

Why not just use fgets and sscanf?

char buf[80];
float n;

if (fgets(buf, 80, stdin) != NULL) {        
    if (sscanf(buf, "%f", &n) == 1)
        printf("%f\n", n);
    else
        fprintf(stderr, "invalid float\n");
}