Question
Hello,  Three years ago I was given a few small plants that were called Midnight Cowboys. The leaves
were low growing to the ground and resembled dandilion leaves but much thinner. The flower had 2 to 3 approx. 5 inch thin stems with small buds.  In the evening the buds would open, unravel and produce white flowers. I can't recall how many petals, but I'm thinking it might have been a single flower, but can't swear to it.
Anyway, they did great and multiplied, but this year, not even one came back. I have tried searching for them on the web but have not been able to find anything similiar. Do you have any idea what they could be?
Thanking you in advance,
Brenda

Answer
Hi Brenda,
Thank you for your question.

What you describe sounds like Oenothera pallida or the White Evening Primrose.  Midnight Cowboy may be a cultivar/hybrid, but plants often go by a variety of names/folk names across the country.  This plant has serrated leaves that bear a resemblance to dandelions.  You can google this and look at more pics.  The seed are easy to come by and easy to start.  Oenothera can and often will disappear after blooming for many seasons.  I have had the yellow and pink in my back yard and they both just disappeared after blooming regularly for several years.  
I hope this helps.
Tom

http://www.perennials.com/plants/oenothera-pallida.html