Can you regain bone density?

Yes. Adequate nutrition and weight bearing exercise, adequate calcium and vit d intake, and some prescription drugs (teriparitide, . ..) can help you regain bone density.
Sometimes. If your loss of bone density has been recent and/or due to illness or immobility you may be able to regain some of the bone you have lost. Women lose bone density with each pregnancy and nursing and then regain it afterwards through mechanisms that are not well understood. By optimizing your nutrition (particularly calcium and vitamin d intake) and weight-bearing physical activity you may regain.