First check that the ethernet card is activated? If not, are modules loaded for ethernet card?
If active, check that you can reach your modem?
Does your modem have the onboard flash memory to get your IP from your ISP?
Some of the following commands may give you some ideas to check. Should be easy to find out what each command is doing ..
#killalll dhcpcd
#ifconfig eth0 down
#dhcpcd eth0
#ifconfig eth0
#lsmod
#netconfig
#/etc/rc.d/rc.inet1 stop
#/etc/rc.d/rc.ient1 start
#dhcpcd
#dmesg | less
#man ifconfig
#cat /proc/pci
#ping 216.239.37.99
#ping
www.google.com
#cat /proc/net/dev
#start adsl
#adsl-setup
#adsl-start
#stty
#/sbin/modprobe xxxxx (to load module)
#/etc/rc.d/rc.inet1 eth0_restart
#ifconfig eth0
#./rc.inet1 stop
#./rc.inet1 start
#ifconfig eth0 up