This is what the spring is supposed to look like when it is properly seated. If you don’t see the end of the spring that’s supposed to bear against the bar, then it has somehow gotten out of position, and that end is behind the bar. It’s not doing you any good there; the sliding part will slide much too easily (no tension at all) when disengaged.
You’ll need a small tool. A jeweler’s screwdriver is perfect. A very narrow-bladed knife is OK, but be careful. You’re going to flip the tip of the spring back onto the front of the bar.
It’s easier to reset the spring if you remove the sliding part from the bar first. To do this, you’ll have to unscrew one of the mounting screws — the highest one, furthest from the fifth string peg. If you have a long bar (3 screw mount), also remove the middle screw. Now the bar is held in place by a single mounting screw, the one nearest the 5th string peg.
note: handle the bar carefully, and make sure not to lose the tiny mounting screw(s)
• Swing the bar up, clear of the neck, and pull the sliding part off the bar.
• Holding the sliding assembly in your left hand, insert the tip of the screwdriver (or knife) into the dovetail slot, behind the tip of the spring, and lift the spring forward
• Here’s the tricky part. Continuing to hold the spring in the forward position, put the sliding part back onto the bar, making sure that the tip of the spring is now in front of the bar, resting on the 45 degree angle, and not behind it. (It might take you a few tries to get this on right.)
• Check to be sure that the sliding part is no longer falling free, but moving under spring tension. If so, replace the screws. If not, try replacing the sliding part again as above.
Would you rather have us fix it for you? If you’re not having any luck with this, or if you just don’t want to bother with it, we’ll fix it for you. Remove the entire unit, bar and all, and send it to us. Be sure to pack it up so that it is protected; the bar is fragile.