mjhjmtu

I've been using Debian GNU/Linux unstable (sid) for five months now without problems. A few days ago my computer wouldn't connect through ens33 (the Ethernet port at the back of my desktop, embedded in), and I had to use an external USB-to-Ethernet converter for internet. When I checked ifconfig -a, I was shocked there wasn't even an entry for ens33 and eth0 refuses to be run ifup or ifdown with.
ifconfig -a result:

enp2s0: flags=4098<BROADCAST,MULTICAST> mtu 1500
 ether 2c:27:d7:43:3f:03 txqueuelen 1000 (Ethernet)
 RX packets 0 bytes 0 (0.0 B)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 0 bytes 0 (0.0 B)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

enx00e04c6803a1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.8.120 netmask 255.255.255.0 broadcast 192.168.8.255
 inet6 fe80::2e0:4cff:fe68:3a1 prefixlen 64 scopeid 0x20<link>
 ether 00:e0:4c:68:03:a1 txqueuelen 1000 (Ethernet)
 RX packets 19402 bytes 22089157 (21.0 MiB)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 13011 bytes 1363539 (1.3 MiB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
 inet 127.0.0.1 netmask 255.0.0.0
 inet6 ::1 prefixlen 128 scopeid 0x10<host>
 loop txqueuelen 1000 (Local Loopback)
 RX packets 230 bytes 128048 (125.0 KiB)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 230 bytes 128048 (125.0 KiB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

How can I add the standard Ethernet interface back?

ErikF's request:
lspci shows

00:00.0 Host bridge: Intel Corporation 4 Series Chipset DRAM Controller (rev 03)
00:02.0 VGA compatible controller: Intel Corporation 4 Series Chipset Integrated Graphics Controller (rev 03)
00:1b.0 Audio device: Intel Corporation NM10/ICH7 Family High Definition Audio Controller (rev 01)
00:1c.0 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 1 (rev 01)
00:1c.1 PCI bridge: Intel Corporation NM10/ICH7 Family PCI Express Port 2 (rev 01)
00:1d.0 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #1 (rev 01)
00:1d.1 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #2 (rev 01)
00:1d.2 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #3 (rev 01)
00:1d.3 USB controller: Intel Corporation NM10/ICH7 Family USB UHCI Controller #4 (rev 01)
00:1d.7 USB controller: Intel Corporation NM10/ICH7 Family USB2 EHCI Controller (rev 01)
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev e1)
00:1f.0 ISA bridge: Intel Corporation 82801GB/GR (ICH7 Family) LPC Interface Bridge (rev 01)
00:1f.1 IDE interface: Intel Corporation 82801G (ICH7 Family) IDE Controller (rev 01)
00:1f.2 IDE interface: Intel Corporation NM10/ICH7 Family SATA Controller [IDE mode] (rev 01)
00:1f.3 SMBus: Intel Corporation NM10/ICH7 Family SMBus Controller (rev 01)
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8101/2/6E PCI Express Fast/Gigabit Ethernet controller (rev 02)

Debian is transitioning to Predictable Network Interface Names.

enp2s0 = Ethernet controller, bus 2, slot 0, first and only function, or in other words, PCI device 02:00.0. And the lspci output indicates it's a Realtek RTL8101/2/6E.

It looks like the RTL8106E at least needs a firmware file or two: you may need to have the firmware-realtek package installed from the non-free part of Debian. (Still available without paying anything, but with some extra copyright and/or licensing strings attached.)

https://packages.debian.org/unstable/kernel/firmware-realtek

ens33 would mean a PCIe hot-plug slot #33 based on information stored within system firmware; did you by any chance disable a PCIe hot-plug support module, thinking it unnecessary, at the time the problem appeared? Or did you do any kernel updates or configuration changes at that time?

A full decoding key for new-style NIC names is here.