Hi ,
It always good to rule out health issues when a sudden behavior change or decreased activity. Birds huge being sick , until they are so sick they cann not ..a fluffed not eating bird is critically ill sbd often 48 hours or less from death .
A good tool to help determine health is regular weight checks , a digital kitchen scale works wonderfully and is cheap . I try and weigh mine about every 2 weeks , daily if I think something is off. Most birds with a health issue will start dropping weight , and you can pick up before other symptoms. Tho not always will they loose weight if it's rapid , and other r reasons.. but it is an excellent way to help monitor.
Being a poop watcher is also helpful. Do not evaluate the first morning poop, as they often look weird and have bilirubin from fasting and holding poop over night.
Evaluating fresh poops during the day , is their a good volume of fecal matter , creamy urates , and only a small amount of liquid urine thst is clear is the standard for many species.
If this is just behavior, anything can have happened to break trust , both known and unknown. I have certainly had this happen for both reasons! Right now I'm currently trying to earn back trust that I completely caused by chasing my green cheek around to return to cage .
If unknown, I try and evaluate the cage,the set up , the diet, the surrounding area , and our daily routine.
Is your bird clipped or flighted ?
If clipped , consider letting him become flighted. We can help with that transition just let us know.
If clipped , provide more ways for self directed behavior, well really this applies if flighted as well. Provide many perches on the outside of cage , connected walkways , hanging perches above cage and dime other parrot hangouts furniture elsewhere. Provide a side shallow dish fur self choice bath daily, I have used serving dishes, class casserole dishes , or larger ceramic dishes made for larger pets. Only about 1-2 inches of water seems preferred for our size birds.
Evaluate toys , GCC prefer easier to chomp and destroy materials. Create foraging fun !
@Emma&pico has do many fun examples she better share !
Get to offering veggies! Dint gir r or starve to eat them . Provide them in many different ways many different kinds expect waste and time before they discover they love them! In the meantime they def enrichment anyway

bell peppers, especially yellow and orange are taken to by many and great source of vitamin A! Any peppers! Even hit ones are great ! Carrots, sweet potato, leafy greens Swiss Chard, mine love rainbow and eat the stalks too. Fruit only about ten percent of daily intake. Grapes only as rare treat . Instead choose blueberries, blackberries, raspberry, mango , pomegranate, strawberry, plums and cherries ( no pits )
Think of this as a life overhaul. All these factors impact behaviors.
And earn back trust , bribes are your friend here! A million hend d out seeds like safflower ( GCC crack) I use popcorn, and other things as well. What ever yours loves. Going up say hi hand treat walk away , like fifty times a day ! Treats to target train. I do a lot of lure , like I show treat and tsp a pet h and ask vine here sweetie! Very near to them at first , like an inch or two then as they get the hang of it I move farther away , until eventually I can move them all over going after those treats.
Be predictable, hav routine ( doesn't have to be rigid)
If only comfort in his spot, work slowly to build his confidence farther from that spot . Like I might have them step up , bring them up to say hi treat and set back down in their spot. Then after a few times of that , I might take 2 steps away give treat and praise them go and set them back in their spot . You gonat their comfort, and keep expanding. At dime point I have the comfortable enough I can set them at a secondary perch area within view of their home spot , and start getting them comfortable st hanging out there. Like taking them to thst perch step off and step back and treat and bs k to home base. Then longer st new perch are sbd bs k to home base