// OnPlayerCommandText

// Command: "/NameSearch" 
ShowPlayerDialog(playerid, 1, DIALOG_STYLE_INPUT, "Name Search", "Enter the account name You wanted to see if exists or not, here:", "Search", "Cancel");

// OnDialogResponse

    // Name Search
    if(dialogid == 1 && response == 1)
    {
 
    new File:players= fopen("Players.txt", io_read); 
    while(fread(players, inputtext)) 
    {
          /*
          I've tried everything, also ******d it billion times and nothing.
          I just need a code here which is going to say if the searched name is registered (exists) or not.
          */
    }
    fclose(players);

    }

new string[MAX_PLAYER_NAME]; // If your file has each registered name in 1 line instead of having 1 per line, then change this.
    new File:players= fopen("Players.txt", io_read); 
    while(fread(players, string)) 
    {
        if (strfind(string, inputtext, true) != -1)
        {
            // Found it
        }
    }