Most likely is that she's had a stroke. The fact that she's had it before and it 'corrected' itself suggests she has possibly had a series of mini strokes and TIAs (Trans Ischemic Attacks).
My Mum has mixed Dementia but I believe the emphasis is on Vascular Dementia and she had several TIAs back in April, following a more significant one the previous August. We called the ambulance the first two times but the decision was made not to take her to hospital as she got very distressed at the thought of going. Also, as she has Dementia, they wouldn't have been able to give her any treatment anyway (they usually give some kind of drug which 'flushes through' the clots in the brain but this is apparently not given in the case of Dementia sufferers - I have no idea why).
Each time she's had a stroke she's had drooping to her mouth and once lost the power to grip with her right hand (she's left handed). And each time these things 'corrected' themselves within a few hours or by the following morning. Once her mouth was back to normal by the time the ambulance crew arrived.
Each time her Dementia has got a little worse after each stroke, resulting in her being very confused and sometimes not knowing who I was. After a few months this has happened less and less, although her general cognitive skills continue to decline.
So it sounds to me like this is the most likely explanation for what has happened to your wife I'm afraid, but you really should seek medical advice.