The most common cause of a lack of balance is vestibular disease. You see it often in older cats, particularly. It's a disease of the inner ear and can cause stroke-like symptoms. You will usually see one sided weakness, drooling, facial slackening and an inability to be mobile. There may even be problems with the eyes where they will look like they are vibrating or watching a tennis match. You might or might not see this.
Your vet can diagnose this and treat it. I really think that you need to and take her in today.