Many possibilities exist,
and depend somewhat on your age, your risk factors,
and the nature of your
symptoms (whether or not the numbness started suddenly or gradually, whether it is a complete loss of sensation or a tingling, whether it is an objective finding on examination or it is only a feeling that you have etc) .
The most concerning cause of weakness on one side of the body is a stroke. Please understand I am not trying to imply you have a stroke, but only that this is one possibility.