You have a lovely old girl, she must have been very well looked after to reach such an age
Sometimes piggies recover mobility, sometimes they don't, it depends on the cause and only a vet can try to find that out. Even if they diagnose the cause there is not always a treatment. For example if piggy has had a stroke there is not too much they can do. It is always better to see a vet than not, for instance they can at least prescribe a little painkiller/anti-inflammatory (like meloxicam) but it is not always possible. So whatever has happened (and depending on your time zone) right now all you can do is help her to be comfortable until she can see one.
She might feel most comfortable being hidden in a little house or cave - wherever she usually sleeps at night to help her feel safe. If she is not able to move easily you can help her by moving her hay, food and water close to there. If she drinks from a bottle you might need to lower it as well as move it closer so she can reach easily. Don't try to encourage walking - if she can walk she will anyway. She may be tired and need to rest.
There is no reason to avoid veggies so some little tasty treats will help her feel cheerful. If eating her hard pellet food is difficult try soaking some pellets in water first so she can slurp them up.
Make sure she is not lying helpless in her own pee/poop... you can switch out bits of fleece for clean pieces, replace wood shavings etc more often, replace soiled hay etc. If she gets poop stuck to her back end just wipe gently with a damp cloth or cotton wool pad soaked in warm water. Don't try to bath her - she won't want to be wet. Gentle handling in case she has pain or is frightened. But keep talking kindly and keep everything familiar and within reach.
Sometimes people use a heat pad or a well-wrapped hot water bottle as a source of gentle heat but it depends on your climate and it is important - especially with a less mobile pig - that she is not trapped next to it. If you put something like this near her she should be able to either move towards it or move away depending on how she feels. I sometimes put the hot water bottle
outside my tray-cages so it just heats up one wall. Then the piggies
inside can choose whether to lean on that warm wall or to move away.
Good luck little girl, your loving owner is looking after you x