How do I mark a character to be selected in C

This is my hangman game code so far. I have a couple of concerns some of which are how do I mark the selected letters to be "SELECTED" so when the user chooses another letter, to not be able to use a letter that he has been used already. And also, I have a losing condition which if met, the game should break with a message however it never breaks.

I have already tried creating another array where I was going to be pushing all of the characters that have been selected. However, it did not work as I was getting errors as I cannot put characters in random positions as the previous slots wouldn't have been populated.

This is my main method:

int main() {
    srand(time(NULL));
    // the randomly selected word
    char *word = generateStrings();
    char sampleArray[strlen(word)];
    sampleArray[strlen(word)] = '\0';
    for (int i = 0; i < strlen(word); i++) {
        sampleArray[i] = '_';
    }
    printf("%s\n", word);
    char *printedWord = printWord(sampleArray, word);
    return 0;
}

This is the method that basically does all the logic:

char *printWord(char sampleArray[], char *word) {
    char letter;
    int stopLoop=0;
    int x = 0;
    int winningSpree = 0;
    int lost = 0;

    while (1) {

        system("@cls||clear"); // clear cmd window every itereation of the loop
        printf("%s\n", word);

        if (winningSpree==strlen(word)) {
            printf("Congratulations! You've won!\n");
            printf("%s", sampleArray);
            break;
        }
        if (lost == 6) {
            printf("You lost!\n");
            printf("The word was: ", word, "\n");
            break;
        }
        if (x == 0) 
            for (int i = 0; i < strlen(word); i++) 
                printf("_");

        if (x > 0) 
            printf("Current stage of word: %s", sampleArray);

        printf("\n");
        printf("Give me a letter: ");
        letter = askforinput();
        printf("\n");

        for (int j = 0; j < strlen(word); j++) {
            if (word[j] == letter) {
                sampleArray[j] = letter;
                winningSpree++;
            } else {
                lost ++;
            }
        }
        x++;
    }
    printf("\n");
    return sampleArray;
}

And this is the method that asks for input:

// ask user for character input
char askforinput() {
    char chosenLetter;
    scanf("%c", &chosenLetter);
    return chosenLetter;
}