Did you let letsencrypt command to modify your Nginx config files? You need to revert them.

In which file?

Obviously letsencrypt modified your nginx config files, it's hard for us to know which files it modified and caused the issue. You can run command "nginx -t" to figure out which config file has issue.

below is my recovered nginx.conf file from a back up location. :
**********************************************************************************************************************************
user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
        worker_connections 768;
        # multi_accept on;
}

http {

        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # SSL Settings
        ##
        ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
        ssl_prefer_server_ciphers on;

        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        ##
        # Gzip Settings
        ##

        gzip on;

        # gzip_vary on;

        # gzip_proxied any;
        # gzip_comp_level 6;
        # gzip_buffers 16 8k;
        # gzip_http_version 1.1;
        # gzip_types text/plain text/css application/json application/javascrip$

        ##
        # Virtual Host Configs
        ##

        include /etc/nginx/conf.d/*.conf;
        include /etc/nginx/sites-enabled/*;
}

#mail {
#       # See sample authentication script at:
#       # http://wiki.nginx.org/ImapAuthenticateW … ePhpScript
#
#       # auth_http localhost/auth.php;
#       # pop3_capabilities "TOP" "USER";
#       # imap_capabilities "IMAP4rev1" "UIDPLUS";
#
#       server {
#               listen     localhost:110;
#               protocol   pop3;
#               proxy      on;
#       }
#
#       server {
#               listen     localhost:143;
#               protocol   imap;
#               proxy      on;
#       }
#}
**********************************************************************************************************************************
Now this one is copied from a backup file.
This one shows error "nginx -t" :

nginx: [emerg] no port in upstream "php_workers" in /etc/nginx/templates/fastcgi_php.tmpl:13
nginx: configuration file /etc/nginx/nginx.conf test failed
**********************************************************************************************************************************

/etc/nginx/templates/fastcgi_php.tmpl is as under
####################################################################
# Template used to handle PHP fastcgi applications
#
# You still need to define `SCRIPT_FILENAME` for your PHP application, and
# probably `fastcgi_index` if your application use different index file.
#
include fastcgi_params;

# Directory index file
fastcgi_index index.php;

# Handle PHP files with upstream handler
fastcgi_pass php_workers;

# Fix the HTTPROXY issue.
# Reference: https://httpoxy.org/
fastcgi_param HTTP_PROXY '';
#########################################################################
"/etc/nginx/conf-enabled/php-fpm.conf" is as below:
#########################################################################
upstream php_workers {
    server unix:/var/run/php-fpm.socket;
}
#########################################################################

Yes I have it as below
srw-rw----  1 www-data www-data    0 May 27 21:28 php-fpm.socket