Did they get hammered by the wind a couple of weeks ago ? That would knock them over and onto the soil. If they are still green just leave them. I doubt you fancy tying them up to supports. If the leaves had changed colour, then that would suggest they were finished growing .. but it is a bit early to have finished yet, there should be at least another 6 weeks of growing left in them. Onions dont need a lot of water, but with the weather we have had I wouldn't be surprised if they they weren't truely stressed.
Mine were sets, planted late Autumn.. they were going really well and looking like they were going to be the best onions ever. Then we had a couple of frosts about 3-4 weeks ago. The whole lot went to seed and formed long 2 ft plus rigid stems with a hollow core. All are still standing, but they have all gone woody down to the bulb .. a real shame as I had 300.