I have had a Low Energy warning on a A319 due to Windshear. Both A/P and A/T were engaged. The easiest thing to do is to advance the thrust levers beyong the climb detent to increase the available thrust (you will only have climb thrust avail when levers are inthe Climb Detent). The amount you increase will depend on how much power you require to get yourself out of the low energy situation.
Don't do what I did and go to TOGA (unless you need to) cause that opended a whole new can of worms.