my nginx server was working perfectly fine and now shows a 502 error

I'm deploying my project, I had no problem until i decided to buy a domain for my digital ocean droplet i added the records from my namecheap domain to my droplet i was working on it then suddenly when i want access to myproject/api it's now showing 502 errors i'll give you all the informations so maybe you can help me with it

1 ) mongoDB atlas status : All good ; i even tried " npm start " to see if DB can't connect but it connects perfectly fine. I refreshed in .env the connecting string, it stills connect with no problems.

2 ) Nginx error.log shows this information i don't really understand, i guess it's the key point : 86 connect() failed (111: Connection refused) while connecting to upstream, client: 164.92.206.160, server: _, request: "GET /api/category/cinema HTTP/1.1", upstream: "http://[::1]:8000/api

3 ) The front is perfectly showing himself on my project domain url it starts showing 502 error when i click something using my DB/server.

4 ) i rebuilded and restarted front & back + nginx multiple times

5 ) i updated endpoints and .env variables to match with my fresh domain and it works, only the API side is breaking

6 ) nginx conf file :

server {
        listen 80 default_server;
        listen [::]:80 default_server;

      

        root /var/www/html;


        server_name _;

        location /api {
    proxy_pass http://localhost:8000;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
}

location / {
    proxy_pass http://localhost:3000;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
}

I tried barely everything and it's still not working as it was used to work.

As the linked thread indicated in comment, probably your upstream server isn't listening at http://[::1]:8000.

Note that [::1] is the IPv6 loopback address (like 127.0.0.1 for IPv4), so if the upstream is IPv4 only, you should explicitly write 127.0.0.1 in config file, instead of localhost.