#define THREAD_LOAD_PLAYER 3
#define THREAD_SAVE_PLAYER 4

new gPass[MAX_PLAYERS][128]; // the string you'll store the password in.

stock LoadPlayer(playerid)
{
    new 
        query[128],
        name[24];
    
    GetPlayerName(playerid, name, sizeof(name));
    
    format(query, sizeof(query), "SELECT `password` FROM `users` WHERE `name` = '%s' LIMIT 1", name);
    mysql_function_query(connectionHandle, query, true, "OnAccQueryFinish", "dd", playerid, THREAD_LOAD_PLAYER);

    return 1;
}

stock SavePlayer(playerid)
{
    new 
        query[128],
        name[24];
    
    GetPlayerName(playerid, name, sizeof(name));
    
    format(query, sizeof(query), "UPDATE `users` SET `password` = '%s' WHERE `name` = '%s'", gPass[playerid], name);
    mysql_function_query(connectionHandle, query, false, "OnAccQueryFinish", "dd", playerid, THREAD_SAVE_PLAYER); // cache is false since you're saving
    return 1;
}

forward OnAccQueryFinish(playerid, threadid);
public OnAccQueryFinish(playerid, threadid)
{
    new rows, fields;
    cache_get_data(rows, fields, connectionHandle);
    
    switch(threadid)
    {
        case THREAD_LOAD_PLAYER:
        {
            if(rows) // if there is rows, then the player is registered
            {
                cache_get_field_content(row, "password", gPass[playerid], connectionHandle);
                // the password is now loaded into gPass
            }
            
            else 
            {
                // if not he's a new player
                ShowPlayerDialog(...);
            }
        }
        
        case THREAD_SAVE_PLAYER:
        {
            SendClientMessage(playerid, -1, "Your account has been saved.");
        }
    }
    return 1;
}