Unfortunately, injuries happen to most people and sometimes you’re forced to take time off. Not all hope is lost though–it’s usually possible to maintain some form of exercise/fitness while you’re rehabbing; it just involves some creative thinking with cross-training. I’ve had to come up with a lot of modified workouts for injuries or weak joints for people in classes and private sessions over the years, so it’s definitely possible to still keep to some sort of exercise.
For most people, the biggest challenge of having to take time off is mental. If exercise is something that you do often, it inadvertently becomes a part of your identity. Knowing that you have to take time off feels like you’re being benched. It can get depressing very quickly, even way before you actually lose physical strength (which takes about 2-4 weeks). So staying positive will probably be the biggest challenge while you’re forced to take some time off. Just remember that it’s temporary and whatever you’re doing will be waiting for you when you’re ready again.
While you can’t run with something like an ankle or foot injury, you can usually still do cardio by via rowing, stationary bike (possibly with one leg), or swimming. Depending on the injury, another example of how you can modify exercises would be burpees: Instead of jumping, step back into the squat position and instead of jumping up, slowly go on your tiptoes while reaching your arms to the sky. You can still do HIIT routines, just look up “low impact HIIT” for ideas.
Weight lifting and/or kettlebells and/or resistance bands are also a good cardiovascular workout alternative (as well as strength training), believe it or not. Obviously, if you have a back injury, I would not recommend you lift weights. If it’s a lower body injury, then this is your chance to work out the upper body and vice versa if you’re dealing with an upper body injury. You may have to lower the weights used while you’re injured and in that case, just up the reps.
Once you do get the OK from your doctor/therapist to get back to it again, take a second to think about how you got your injury in the first place. If you ended up with your injury because you were pushing yourself too hard and never taking a day off or if you weren’t wearing the proper gear, etc., think about what fixes you can make to avoid injury in the future. That might also include proactively doing mobility/physical therapy exercises. The takeaway is to try and actively prevent getting injured again so that you can keep doing what you’re doing!