In all likelihood, the case will just be put on hold until he returns. You could hire a different lawyer or request his office/firm have another lawyer there handle it, but if you want to stick with that same lawyer, you'd have to wait until he returns in good health.
In the interim, you may want to notify the other party (and possibly the courts too) that you are still proceeding with the case, though it will be temporarily on hold on account of your lawyer's health. If you can't get hold of anyone from his office to do this for you, then you may wish to consider drafting a letter yourself and including the email saying he's on medical leave, then providing this letter to the other party and courts so that they know the situation and won't try to take any significant steps in the down time.
Hopefully that is of assistance.
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