In some cases, cats may see their litter boxes as private spaces, and your cat may choose to stay in the litter box in order to get a sense of security. This is particularly common with enclosed litter boxes, which mimic the quiet security of other spaces cats like, like closets and cardboard boxes.For example, when there are some visitors in your house or thunder , your cat may be frightened or hide in the litter box.

It may be normal to hide in the litter box suddenly like above, but if your cat is suddenly spending more time in the box everyday, you have to pay more attention on your cat’s health.Cats with diabetes, urinary tract problems, difficulty urinating, or kidney problems will sometimes stay in the litter box.
There are three common medical conditions to consider:

• Feline urethral obstruction (FUO)
  This condition, which occurs primarily in males, is commonly referred to as being “blocked.” A urethral obstruction occurs when debris (usually consisting of mucus and crystals) accumulates in the bladder and comes together to form a plug. Without treatment, urethral obstruction will progress to a life-threatening emergency and death.
• Pandora’s Syndrome
  The symptoms resembled that of a human urinary tract infection, like straining to urinate, passing very small amounts of urine and often having blood in the urine. However, researchers have found that in cats, this condition is very rarely caused by a bacterial infection. It has been associated with obesity, inactivity, but most importantly, STRESS! It usually achieved by addressing the associated causes of obesity, inactivity and stress through environmental enrichment.
• Constipation
  Constipation is more often seen in middle-aged to geriatric cats (around 8+ years of age). The average cat defecates at least one time every 24-36 hours. A healthy cat bowel movement should be well-formed and moist enough that litter will stick to it. If you are seeing small pieces of dry stool, or stool less frequently than every 36 hours, your cat may be constipated. In addition, constipated cats may strain in the litter box and may even vomit as a result of straining. There are some tips and home remedies for constipation in cats:
  1. Increase Water consumption
  2. Try a New Diet
  3. Help Your Cat Maintain a Healthy Weight
  4. Increase Exercise and Enrichment
  5. Minimize Stress and Anxiety
  6. Add More Litter Boxes
  7. Try Fiber or Probiotics
  8. Ask Your Vet About Over-the-Counter Laxatives
  9. Monitor Your Cat for Constipation

If there is any situation, it is recommended to contact your veterinarian as soon as possible! Take care of your cat’s health.

Source:

https://docandphoebe.com/blogs/the-catvocate-blog/the-top-three-reasons-that-your-cat-is-suddenly-spending-more-time-in-the-litter-box

https://www.popsugar.com/pets/why-is-my-cat-sitting-in-litter-box-47940045

https://www.pawtracks.com/cats/cat-lying-litter-box/

https://www.greenmatters.com/p/cat-sitting-in-litter-box

https://www.petmd.com/cat/care/8-ways-help-your-constipated-cat