So I don't think its sick, I just think its too weak/long from not enough light and has been reaching. I would think your other ones will fall over soon too just because they aren't meant to grow upright anyways without support.

All of the plants are very leggy due to stretching too far for too little light. The stems should only be no more than 1/2 that height. Check out the FAQs here on how to grow seedlings indoors. Supplemental lights are required. Window light alone is never sufficient for young seedlings.

Cucumbers are best direct seeded in the garden, not grown with transplants. Not only do they not transplant well much of the time but they grow far too fast for most indoor set-ups to cope with them well enough until they can go to the garden. They tend to stunt if transplanted after the 3-4th true leaf develops.

Dave.

I started a few indoors, though plan to direct seed the 8-10 cucumber plants I plan to have. Just wanted to study their growth process, and they do grow faaaaast. Root bound very quickly.