Complete lack of energy. What to do?

[Loki]

Anyway.....periodic extreme energetic fatigue, yes . . I am feeling it, too. I like all the suggestions of light exercise. 

Think of a white ray of light coming from the centre of the planet and powering each of your energy centres. If you cannot "see" the colour of one of your centres it might be blocked.
I had a time I could not see my red centre and I imagine it as being locked in a dark rock. I was able to blow the rock to pieces and the red light started shining. Intention is very important.
Imagine that all the dust and dirt falls from your centres into the white light under your feet and your centres are shining clear. Do this few minutes every day. Don't forget to ask you higher self for permission and to accept that light and healing service to self. It is not and STS service. It is just your duty towards yourself.

Once you master this first exercise you can start invoking also the adept ray of light from above thorough the crown chakra. The two rays (above and below) can then meet at green chakra which will be used to power and heal your different bodies.

I hope this helps.