3 Answers

Lack of energy can be caused by so many contributors like not eating not drinking enough fluids not getting enough exercise lack sleep going to bed late and if you are not doing any of these things I sujjest you go and see a Doctor you could well be anemic or have a under active thyriod gland good luck

Sometime lack of energy for me is caused by my sitting around too long.

The less you do, the less you feel like doing.

The more you do, the more you feel like doing.

This is by no means an answer to your problems. I'm just saying what happens to me sometimes.

Make sure you're drinking enough water too.

Possibile causes are not enough sleep, loss of appetite, stress, side effect of medication.  Advisable to see your doctor.