I'm unable to walk upstairs on my own. How do I figure out what's wrong?

Stroke?? It really depends on your age and symptoms. It could be balance or weakness caused by stroke or neurologic problem. If it is a particular injury, a certain area would be painful. Is there pain. If it is general weakness, progressive, muscular dystrophy may be the problem. 33 years is awfully young not to be able to do stairs, are you injured?