difficult to answer you without more details!
try the following
install dhclient if it not already present on your system:
# pacman -S dhclient
edit your /etc/NetworkManager/NetworkManager.conf and change like this:
[main]
# ...
dhcp=dhclient
# ...
please refer to
https://wiki.archlinux.org/index.php/NetworkManager# Problems_with_internal_DHCP_client