Stress can deplete your energy level completely, leaving you unable to train properly. Some possible causes may be traffic, work, marriage, deadlines, lack of sleep, overwork and much else.
A Homeopath might recommend a Combination of all Tissue Salts which you can have made up (online order) by a Homeopathic manufacturer at a very reasonable price. They come in one small pillule and you can take one a day for a few days. If there is going to be a delay, you can start with Homeopathic Kali phos (Kali phosphoricum), which is freely available in numerous pharmacies.
If you want more, you can also take some Cit ac (Citricum acidum) which boosts oxygen useage and improves energy levels. In extreme need you can add Phos (Phosphorus) for general re-generation of the entire system, and Sepia if there is a heavy personal emotional component.