No-one seems to have a save handy -- if you're still having problems, perhaps you could go back to an earlier save and replay this part through, that might unstick you.
Be sure not to miss any of the steps. Here's the extract from MaG's walkthrough:
Place the reflector on the stand at the middle of the green laser's path.
Yellow laser: Turn the stand that is the source of the yellow lasers at top left. The yellow lasers now go through the green laser path.
This bends the yellow lasers to the laser concentrator at left. See that the color of the concentrated laser beam is now yellow-green.
Green laser: Turn the right stand that sends out green laser to point at another concentrator. See that the emitted light from the concentrator is red-green.
Click on the blinking red light on the box at bottom left. It is emitting a signal. Click to turn that red light off.
Other concentrators and blinking red light box: Go to the box left of the computer monitor and turn it off. That turns the gas tank on the ceiling and the lasers off.