I have been diagnosed with severe vitamin d deficiency. i am really tired and don't feel i can't go back to work, how do i get my energy back?

Take Vitamin D!: I assume your diagnosis was made based on a blood test showing a very low level of 25-hydroxyvitamin d also known as calcidiol. If that is the case, and assuming that there is not a problem with you absorbing d from the gut, you can take 2000 unit of vitamin d supplements a day for a few months and then drop down to about 1000 units a day. You should consult with the doctor who made the diagnosis.

Vitamin D: If you were diagnosed with "severe" vitamin d deficiency, your 25ohd level would usually be <20 ng/ml. For this, your doctor can treat you with 50, 000 iu d3 orally once per week for 6-8 weeks. This could be followed by taking 1000-2000 iu per day, with a re-test @ 3-4 months to see where you are. If the reason for vit d deficiency is poor absorption, your needs may differ. Ask your doctor.