Is it possible to reattach the v-brake spring/pin arm? I was fitting new brake pads to my (mini) v brakes and stupidly pushed one of the arms very far inward to get easier access.

The spring (actually I'm not sure I mean spring, the pin/spring that pushes the arm back from the rim) either popped out or snapped, I couldn't tell which.

I couldn't really tell where it was meant to reattach or screw back in. Does anyone have a diagram or such showing how it should attach? Or can anyone explain how it should be attached?

I do not believe it's a model with multiple holes for different tension but I could be wrong.

It is completely loose now and the brake arm has no tension.

The brakes are very similar to these Tektro RX5 brakes unfortunately the manual on that page doesn't mention adjusting/refitting the spring.
I'd start by unhooking the inner cable, like you're about to take the wheel out.

Then unscrew the big silver bolt that goes into the brake boss on the chainstay(rear) or fork(front)

Slide the brake arm off and inspect the spring.  If spring is in more than one piece, you'll need new ones.  Replace both springs to keep the tension close to even.

If everything looks okay, undo the good brake arm carefully and inspect how it goes together.  Reassemble the bad one repeatedly till you get it working right.  

Don't forget to use blue locktite/thread locker on the two big bolts.