Premature Graying of Hair. Reversible?

Since 2-3 weeks suddenly I'm noticing a lot of gray hair on my head. I don't believe this is genetic as a lot of hair turned gray suddenly. Before this I only had 1 or 2 gray hair at max. Here's some background: I started dieting and exercising around 6 months back. I eat 2 home cooked meals each day, have tea twice and a protein shake once a day. I also exercise 5 days a week. During the last 6 months I have lost around 10-15 kgs. Also, a few weeks back I restricted my calories extremely, eating below 1000 calories per day. I noticed the hairfall and gray hair after this. Now that I'm back to my normal diet, the hairfall has almost stopped but the gray hair are still there. I'm also not getting enough sunlight as I am indoors everyday. After researching on the net, I believe possible causes could be a deficiency of some vitamin or stress due to extreme weight loss. Please guide me on how to proceed forward. Also, if this is not genetic, is this graying of hair reversible?