However this is not an excuse to miss training.
As an athlete you should look to your way to be a champion as a long run: sometimes you run, sometimes you walk slowly, sometimes you even crawl, but you will reach your final goal ONLY if you keep chasing it every single day.
Despite that in combat sports we can not fully replace the partner workout and sparring, with little creativity we can still train absolutely everywhere with little or even no equipment.
Injured? Just go and do your skill training in class!
Not able to go to class? Go to gym and do strength training! Every combat athlete knows that in competition, strength is also a skill!
Unable to go to gym? Do bodyweights, calisthenics or just run! If you missed a skill session think about compensating it with strength and conditioning!
No timer? Go for free shadow boxing, feel and listen to your body, until you feel your hands loose and bursting combos with power and speed.
Sometimes the free sessions by your own are so fun, refreshing and free of pressure, that might help you overcome a plato in your routine.
At the end of the day being creative, beating the odds and imposing your will is all the fighting sport is about.