I went through something kind of like that with my sprained ankle a bit ago (it's still not quite up to snuff for doing high impact stuff but getting better) I discovered you can still do lots of things - crunches, seated leg lifts, leg raises, "women's" push ups..... Even sitting in a chair and swinging your arms around will get your heart going a little faster - I did a ton of "chair dancing" and rocking out in the car
No, it's not quite the same as your regular workout, but keeps you from getting completely out of the habit; and the way I look at it - any moving is better than no moving.
Hope you feel better soon