What can cause someone to lose their energy ?

Answer 1

Renal's right - it could be practically anything. You don't provide enough information (frequency, duration, intensity of the energy loss, symptoms that have come with it, etc.) to narrow it down at all.

If it's just a loss of energy and it's started only recently, it could be seasonal affective disorder. I know a lot of my friends are starting to feel a bit run down now that we're getting less sunlight.

http://www.ncpamd.com/seasonal.htm

http://en.wikipedia.org/wiki/Seasonal_affective_disorder

(Those are just two sites. Run a search on Seasonal Affective Disorder and you'll find lots more!)

If there are other symptoms, you should probably check with your doctor.

Answer 2

Lack of adequate sleep, so often I see people who complain of feeling tired all the time and I refer them to their doctors for sleep studies and they ultimately come back with a diagnosis of obstructive sleep apnea.
B-12 taken sublingually will also help to boost energy levels.

Answer 3

Practically any illness going. Anything from a cold through to cancer.

Answer 4

physical acitivity