Lens cap on is a sure fire way to get dark pictures too
The best thing to do is take a meter reading and compare to what the camera gave you.
Now, I'm going to take an educated guess...
According to the Kodak chart I have, subjects lit by streetlight should get 1/15@f/2 on 400 ISO.
On the Holga, taking an f/11 as the average aperture (the old ones are at f/13), it would need to be exposed at 1/4 second on 3200 ISO film.
Considering the holga is clicking at minimally 1/60, that would be a 4 stop push... big.
Holgas were obviously never intended to be used by low light.
Just use the cardboard computer on
http://www.squit.co.uk/photoThey have a lot on information printed on their card.