Having gone through the roughest patch in my life in the last year, I experienced this. The one thing that killed my productivity more than anything else was a mixture of A) Feeling like work was meaningless and questioning why I worked so hard in the first place and B) Sleeping 3 hours a nigh at most mixed with only eating a few meals a week.

Mostly it just took time for me to get back on a semi-normal routine, meditation, exercise, nutrition and rest are really really key factors though.

I hope things get better for you soon.