If you started receiving 0 experience or 0 gold in battle it is because we have a daily experience cap. This means that your character has reached its daily limit of experience or gold. If a character goes over the limit, the next battle reward will be zero gold and/or experience. The tracking is separate for gold and experience, so someone farming for gold may keep earning gold after they hit the max experience cap.
The limits are in place primarily to discourage cheating. Low level characters can level up several times a day, and higher level characters can level up twice a day.
The experience and gold caps are 10% higher for X-Guardians because of the X-Boost.
The daily reset time is at Midnight, server time. After midnight, you can start gaining experience and gold again.