I am sorry to hear you are worried about your cat. My name is Rebecca. I am a veterinarian and will do my best to help.
How old is your cat? How long since she last ate? Is she drinking at least?
If you look at her gums, are they pink in color or do they seem pale? Bright pink gums is a good sign; pale gums indicate she might be seriously ill. If she is a black or gray cat, her gums may have pigmentation, and be hard to assess.
Since I can't see and examine her, knowing her age, if she is vomiting, if she is drinking, and how long she has not been eating would be helpful. However, in general, cats tend to be secretive about not feeling good, being in pain or being sick. They will hid their symptoms, and then when they feel so bad they can't hide it any longer, they "suddenly" stop eating.
If she is not drinking either, she is in danger of dehydration. Try to encourage her to drink; offer tuna juice or chicken broth or milk with water added.
She could have so many things: kidney disease, infection, cancer if she is old, heart disease, liver disease, fever. The best place to start to find out where the problem lies would be a thorough examination by her veterinarian: taking her temp, palpation her internal organs and lymph nodes, listening with a stethoscope, feeling her pulses, looking at her eye, checking her reflexes. She may simply have a fever and need antibotics; if she is older she might have a more serious condition.
Let me know if this helps and what other questions you have.