I will try to answer to the best of my ability. If you need more clarification or have info to add for me, I will be back on line after 3pm this afternoon (PST).
One of the most common reasons for lethargy and inappetance in older cats is renal disease (problems with the kidneys
). Toxins start to build up in the blood (uremia) and they can actually cause irritation to the point of ulcers in both the mouth and all along the inside of the digestive tract (stomach, intestines). If ulcers occur in the mouth, infection easily follows.
Only a blood test and physical examination can say for sure if the kidneys are causing her problems, and if not, what else might be.
Although she may have been stressed by your absence, in itself this would not cause her problems. Stress, however, could lead to her going off her food
and water, and start her becoming dehydrated. This can exacerbate pre-existing kidney problems.
Your veterinarian can administer fluids either IV or under the skin, and give her anti nausea/appetite stimulant medication which can be a tremendous help to how she feels. While kidney disease may not be curable, it is very often well-treatable, with good quality results extending lifespans.
Sometimes cats stop eating for reasons like dental problems (i.e. infected teeth which require extraction).
See the following for more info on kidney problems in cats:
Hope this is helpful.