How can I get multiple bullets on screen?

You really need to have a separate class for your bullets! It will make everything 1000x easier to manage, render and ultimately to answer your question, the ability to keep adding in new bullets when the space bar is held down.

Your class can be as simple as the following (source):

You could use vectors to create them dynamically.

Example:

#include <vector>
using namespace std;

class CBullet
{
public:
    CBullet();
    ~CBullet();
};

vector<CBullet *> Bullets;

Then just use something like:

void Shoot()
{
    Bullets.push_back(new CBullet());
}

And loop through them:

void DrawBullets()
{
    for (int i=0; i<Bullets.size(); i++)
    {
        //do stuff here
    }
}

But I guess an array of bullets would work just as fine. Personally, I prefer to use vectors.

You will need to add some form of update function to the class to manage the movement and ultimately the collision of your bullet, and if you look at the thread it also shows you a bit further down how to remove bullets from a vector array.

But as a word of advice;

1. Get rid of the magic numbers, http://en.wikipedia.org/wiki/Magic_number_(programming).
2. Your naming convention, your variable names should as clear as water to understand, "once2" & "frame2 doesn't mean anything in my eyes without a comment or knowing what it's purpose is for. In-fact i can't see personally why you need that last if statement you have. If i could hazard a guess it looks like to be some form of reloading code? As to an example of making your variables clearer, lets take the spacebar boolean you have. Because of your comment, i know that it's changed when the space bar is pressed. To make it clear as crystal however, you can simple change spacebar to something like "hasSpaceBarBeenPressed".

To answer your question:

It seems that you only have 1 instance of your bullet, which gets position at the first time the spacebar bool is true. It seems then that because once2 is incremented and the spacebar value is still equal to true, the only time that your 1 bullet instance you have will return to be re-positioned will be when once2 == 250 which then you reset that once2 value to 0 and then the spacebar bool to false.