From your query it is not clear whether CBI issued summons to you in your name. You are only mentioning your Office asked you to give Witness. It does not happen like that.
If something happened and a person has some material facts about the happening, CBI when they investigate the case, they do ask some people connected to depose witness before the CBI Court. If a person is issued with Summons, it is his duty to attend the CBI Court. Health issues if there, you can inform the CBI Lawyer about your incapacity for that date and should attend the CBI Court on the next date intimated to you.