I found my cat dead yesterday at the foot of the closet. She looked peaceful but I really am sad and wonder w?

I found my cat dead yesterday at the foot of the closet. She looked peaceful but I really am sad and wonder what I could have done to prevent her death. I had her for 4 years and I know she knew my kids & I loved her. I dont know how old she was because she was given to me by her previous owner who became too sick to care for her and has since died. She was a medium sized adult brown & white tiger indoor cat. She ate dry food daily, and wet food about 3x per week, and drank water daily. I believe she may have been sick because she did meow more lately at night unexplainely even when her food was full. I thought this was because we recently moved to a small one room apt from a 2 bedroom house and was missing the roaming space. Occasionaly over the past few months she would spit up, but I always thought it was hairballs or too much water, because it was basically clear, and she didn't seem distress. I notice the past few days she did not eat her food, only the water. She seemed fine though, no indication to me that she was about to die. I really feel like I failed her. She was my first cat and she made me a catlover and pet lover. This seems sudden to me, she looked so peacefull but it haunts me that I didnt take her to vet or something. Might she of had an infection or something? She wore a flea collar and never seemed to be botherd byfleas or ticks. There were ants in my apartment that I often removed from her food dish. Any thoughts of what caused her death?