Mom has early onset dementia and recently over the past few weeks she has stopped eating. She says she can't and has no appetite. She complains that her stomach hurts and she feels gassy but she just won't eat unless forced to. The only thing we have managed to get in her is some soup broth and juices. She is 87 in a nursing home.
Just be patient. Try rubbing a bit of her food on her lower lip to see if she will taste it. Perhaps tasting it will bring forth her appetite. Try shakes as suggested. Try soft foods, make a few favorites to try.
There is also acid reflex issues, where fruit drinks, chocolates, tomatoes will bring up acid from the stomach which will cause a burning effect in the throat.
And as people age, they lose their sense of taste so only sugary items are wanted. My Mom was like that, most of her grocery list were deserts.
Hopefully the nursing home will find the mystery. Oh, my Mom wouldn't drink Boost or Ensure unless it was put into a cup of ice.
Early in my husband's dementia he went through a period like this, saying he had no appetite, his stomach hurt, thinking of food made him nauseated, etc. His geriatrician prescribed something to help his appetite. It didn't solve the problem entirely but it did get him through this phase and he eventually didn't need those pills.
I went through something similar. I didn't have dementia; I knew I had to eat. So I forced myself to eat a narrow range of items I could tolerate. After losing 40+ pounds and getting weaker and weaker I went to an ER and was diagnosed with diabetes. Treating that solved the problem!
I am certainly not suggesting that your mom has diabetes! But since there can be several causes for sudden unwillingness to eat I would want a doctor to examine Mom. Then, if it is the dementia as we all suspect, perhaps the doc will prescribe something to help her appetite. And if something else is wrong, that can be addressed.