Why is my leapard gecko sick and dying?

ive had these leaperd geckos for like 3 or 4 years. all of the sudden one of them died. she was all mal-nurrished, but for no reason. they had plenty of food and water and they've been happy and fat and totally fine. then all of the suddent the one just died when i was on a trip for a month. the people who watched them for me did everything right, but for some reason her eye was all shut and she never opened it and she could never shed correctly. leaving it all over her face. i was totally confused. now the other one is doing the same thing!! she cant shed correctly and she cant open her eyes. they might just be dieing of old age, but their average life expectancy gives them another year to live and now the second one is diying just like the first. how can i help her and what is wrong??? thank you so much for ur answers