As you can already see, the saddle is out of position (note the head of the saddle position screw is not against the bridge plate as it should be.
You just need to find a replacement spring. You might find something that will work at a hardware store or at an online or brick and more music instrument retailer.
How do you fix it? Strip it down, and put another spring in! However, when the saddle is in the correct place, so it's intonated correctly, and it may even be there now, looking at it, tighten up the screw until its head just reaches the metalwork. At that stage, it could be left for ever, and not affect the intonation or playing of the guitar.
As others have said, springs are available - even one from a ball-point retractable pen could do the trick!
Once the string is present and tensioned, I don't see the spring making a difference. It's just nice for setting up the saddle "offline" and not having it rattle around (and rearrange itself) while no string is there.
It also avoids having the string pressure on the maggot screws temporarily stop the fixture from pulling out fully and then later adjust itself with a jerk when tensioning the string.
But the spring action is nothing that you can't replace with manual action on setup and tensioning, and afterwards it's no longer necessary.