If the judge notices the problem immediately, the bell will be rung and you can stop or circle until the obstacle is rebuilt and the bell is rung again to signal you to resume your round.
If you notice the problem before the judge does, stop and point in the direction of the jump. The clock will be stopped and an adjustment might be made to your time to account for any delay. Wait until the bell is rung before you take the jump.
Here is the official EC rule:
FEI ARTICLE 233 STOPPING DURING THE ROUND
1. In the event of a competitor not being able to continue his round for any reason or unforeseen circumstance, the bell should be rung to stop the competitor. As soon as it is evident that the competitor is stopping, the clock will be stopped. As soon as the course is ready again, the bell will be rung, and the clock will be restarted when the competitor reaches the precise place where the clock was stopped.
2. If the competitor does not stop when the bell is rung, he continues at his own risk, and the clock should not be stopped. The Ground Jury must decide whether the competitor is to be eliminated for ignoring the order to stop, or whether, under the circumstances, he should be allowed to continue. If the competitor is not eliminated, and is allowed to continue his round, the scores obtained at the obstacles preceding and following the order to stop will count whether they are good or bad.
3. If the competitor stops voluntarily to signal to the Ground Jury that the obstacle to be jumped is wrongly built or if due to unforeseen circumstances beyond the control of the competitor, he is prevented from continuing his round under normal circumstances, the clock must be stopped immediately.
3.1. If the dimensions are correct and the obstacle in question has been properly built or if the so-called unforeseen circumstances are not accepted as such by the Ground Jury, the competitor will be penalized as for stopping during the round (223.1) and the time of his round will be increased by 6 seconds;
3.2. If the obstacle or part of the obstacle needs to be rebuilt or if the unforeseen circumstances are accepted as such by the Ground Jury, the competitor is not penalized. The time of the interruption must be deducted and the clock stopped until the moment when the competitor takes up his track at the point where he stopped. Any delay incurred by the competitor must be taken into consideration and an appropriate number of seconds deducted from his recorded time.