I am assuming you are not on any medications and are otherwise healthy. As you mentioned no other symptoms, I am assuming you only have no energy.
There are several options to look into. You should have bloodwork done looking at your CBC (blood count), ESR (inflammatory marker), TSH (thyroid
test), vitamin D
level, CMP (complete metabolic panel), and lyme Ab (if you live in an area where there is lyme).
If all these are normal, and you have no other symptoms (no rash, belly aches, fevers, etc), then take a look at how you are sleeping (are you getting up at night to urinate or having trouble going to sleeP) and consider checking with your doctor for sleep apnea (loud snoring, stopped breathing at night). If you sleep well and just have no pep, then consider having regular daily exercise (about 30 minutes aerobic activity). This releases good hormones in your body called endorphins that give you pep.
Finally, if none of the above are helpful, then consider your environment and emotional health. Are your stressed financially, at work or at home? These things can all contribute to no energy.
Finally, there is no universal medication or supplement that will give you energy. Caffeine can give you some temporary energy, but this is not a medication. I hope this helps you. Good luck.