No matter what the nature of your job is, there will come a time when you're too stressed out from work that you all you want to do is lie in bed and sleep with your clothes on.
In times like these, you have three choices: First, you can 'abort' the plan of working out and save it for another day. Just make sure that it is within the same week so that your exercise program will not fall through altogether.
Second, you can tough it out and do your exercise routine despite being dead-tired. This should test just how far your body can take you.
Third, you can make adjustments to the exercises that you need to do for that day. Instead of having a 30-minute routine, you can probably cut the time in half and save what's left for another day - when you're up to the task.