Looks like there is a serious root problem going on. Not only leaf dieing but also branch dieback. Where are you located? Did you had it outside and got hit by frost?

Light Your tree qualifies for the trash can light

Clamp. lights with plant in corner of room. cardboard painted gloss white will reflect light shining from other 2 open sides. Rotate plant daily. Check for and fix possible root problem first.

Pulling air through the roots can help a lot with soil that holds to much water for winter time care

Steve

Lemon trees are very cold sensitive and it you had it out near or below 0C you probably have a dead tree now. (looks dead to me) All the light in the world can not bring it back

Wait and see. It probably is not dead. The frost killed all the leaves but the larger branches will leaf out later to much later.

Steve

Put it in a south facing window. Water only when the top two inches of soil feel dry. Remove dead branches. (Check to see if they are dead by scratching the bark: if green underneath, branch is alive.) It has a slim chance of survival but may still make it.

If the foliage is receiving direct sun through the window, then the container should also be receiving the same sun light. In an effort to remain cool the leaves are transpiring at a high level due to the sun's rays, but the roots setting in the shade are functioning at a reduced level and are not able to supply the leaves with enough water to replace that which is lost through transpiration. The only way left for the tree to cool down is to discard its leaves.