It is amazing how easy it is to get sidetracked on a goal or project. Sometimes the cause is small, for example needing to run an errand during your normal workout time. Then the next day rolls around and the "need" to run more errands in that same stretch of time becomes easier to justify. Finally, that time somehow switched from workout time to errand time.
Other times there are legitimate, longer period interruptions such as illness, changes at work, or other lifestyle changes that alter the way you spend your time either for long periods or permanently. Not much can be done about these events except how we decide to RE-prioritize around them.