First things first. You need to make sure your cat eats something. Try to keep it away from the other cats incase what ever your cat is sick with is contagious.
Even if it isn't contagious, you don't want the other cats bugging him/her. Make sure your cat gets some water to drink and just sit with him/her. If there is no change within a day or two, try your best to convince your parents to take him/her to a vet.
Just make sure you take good care of your cat. Are there any specific symptoms that you have noticed besides the groaning? Has she/he thrown up or anything unusual like that?
You shouldn't have pets if you are not going to be able to afford or have the responsibility to take them to the vet. In the meantime, the only thing you can do is monitor the cat, but without knowing what is wrong, leaving it could just result in more complications and result in an even more expensive vet bill.
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