There will be times during your workout career that you will lose your endurance.
Maybe you worked with a personal trainer for years and then finances got tight and you had to cut back on expenses and then got out of the habit of exercising altogether.
Maybe you got into strength training and forgot about the importance of endurance training (until that day that you showed up to play pickup basketball and turned out to be the most out of shape person there).
However it may happen, endurance can easily be gained back but it requires time and consistency.
The main thing at first is to work into it slowly and not push the intensity up too much. Try to keep the heart rate at a low/moderate level for a few weeks while you steadily increase the time spent walking, jogging, biking, or whatever you have chosen to do.
It is critical that you don't overdo this by diving it head first and pushing yourself everyday. You will not make progress like this! Your body must have a chance to adapt. Taking every other day off at first can be a good way to achieve this.
The main thing here is to listen to your body and let yourself progress in your endurance at a steady unhurried pace. Remember the tortoise and the hare. Fitness is not a race. Its the consistent person who wins...not the person who is "on again, off again"