eth0 is for the wired connection
wlan0 is for the wireless connection
For configuring a network connection, if it hasn't already been completed automatically, there is a few ways you can go about it from the command line and from the GUI (though I dont have that information handy at the moment)
If you are missing the files in /etc/sysconfig/network-scripts/ then you have to tell nmcli to create the connections, it helps to do checks before you actually start making connections.
Starting with an nmcli dev show to make sure you can see all the hardware with nmcli
[root@linux-repository ~]# nmcli dev show
GENERAL.DEVICE: eth0
GENERAL.TYPE: ethernet
GENERAL.HWADDR: XX:XX:XX:XX:XX:XX
GENERAL.MTU: 1500
GENERAL.STATE: 100 (connected)
GENERAL.CONNECTION: eth0
GENERAL.CON-PATH: /org/freedesktop/NetworkManager/ActiveConnection/1
WIRED-PROPERTIES.CARRIER: on
IP4.ADDRESS[1]: X.X.X.X/24
IP4.GATEWAY: X.X.X.X
IP4.ROUTE[1]: dst = X.X.X.0/24, nh = 0.0.0.0, mt = 100
IP4.ROUTE[2]: dst = 0.0.0.0/0, nh = X.X.X.1, mt = 100
IP4.DNS[1]: X.X.X.X
IP4.DNS[2]: 8.8.8.8
IP4.DNS[3]: 8.8.4.4
GENERAL.DEVICE: lo
GENERAL.TYPE: loopback
GENERAL.HWADDR: 00:00:00:00:00:00
GENERAL.MTU: 65536
GENERAL.STATE: 10 (unmanaged)
GENERAL.CONNECTION: --
GENERAL.CON-PATH: --
IP4.ADDRESS[1]: 127.0.0.1/8
IP4.GATEWAY: --
IP6.ADDRESS[1]: ::1/128
IP6.GATEWAY: --
then you want to see if you can see already configured connections with nmcli con show
[root@linux-repository ~]# nmcli con show
NAME UUID TYPE DEVICE
eth0 0c7ab467-dbdf-41c2-b3a3-882760879594 ethernet eth0
once you have the device name from nmcli dev show just add a generic connection with nmcli con add type ethernet con-name <dev name> ifname <dev name> and this should add a dhcp connection to your wired connection to help get you going to be online again; for wireless, I recommend checking out a nmtui tutorial.
[root@linux-repository ~]# nmcli con add type ethernet con-name eth0 ifname eth0
Connection 'eth0' (058aac84-c024-46bc-9847-b6e8044fb9c6) successfully added.
NMCLI & NMTUI
CentOS 7 Network Configuration Using nmtui Tool here is an CentOS tutorial on nmcli
RHEL 7 nmcli [this is pretty much the same as the nmcli for RHEL 6, Centos 7 & 6]
also can run man nmcli the manual for the tool is also on the system.
ifcfg
How To Configure Static IP On CentOS 6
an example of setting up a network via the ifcfg files, you may have to add the NM_CONTROLLED="no" if you dont want network manager to mess with it.