Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Dan loved playing chess but he just couldn't anymore.  He had been debilitated by what?
headaches
A stroke or an injury. (One of these two possibilities is most likely, but without more information it's impossible to know for sure.)