Upgraded to Debian 9.12, now have no network connection

When I upgraded my cloud server from Debian 8 to Debian 9.12, I lost all network connectivity. I can neither ping the server nor ping anything from the server.

If I try to add a default gateway, I get SIOCADDRT: Network is unreachable

If I type netstat -anr, I get an empty list

There may be other obvious things I should try, but I am clearly not that knowledgeable.

Thanks for any help or guidance.

ip a
1: lo: <LOOPBACK,UP,LOWER_UP. mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever

2: ens3: <BROADAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
     link/ether 00:16:3c:4e:a4:c8 brd ff:ff:ff:ff:ff:ff

/ect/network/interfaces contains:

source /etc/network/interfaces.d/*
#The loopback network interface
auto lo
iface lo inet loopback

#The primary network interface
allow-hotplug enp0s3
iface enp0s3 inet dhcp

There are no files in /etc/network/interfaces.d/*