If you can’t reach (eg ping returns nothing) your server from outside network, check your firewall rules at first. On CentOS it is simple:

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$ sudo iptables -L
 
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT icmp -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:http
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ssh
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
 
Chain FORWARD (policy ACCEPT)
target prot opt source destination
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
 
Chain OUTPUT (policy ACCEPT)
target prot opt source destination

If you don’t see any suspicious firewall rules, use tcpdump utility to check if server actually receives any ping packets.

Check that server receives ping echo requests (see http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol for ICMP control messages):

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$ tcpdump -nni venet0 -e icmp[icmptype] == 0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on venet0, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
10:38:53.926767 Out ethertype IPv4 (0x0800), length 76: 172.245.215.48 > 46.246.9.143: ICMP echo reply, id 1, seq 154, length 40
10:38:54.937709 Out ethertype IPv4 (0x0800), length 76: 172.245.215.48 > 46.246.9.143: ICMP echo reply, id 1, seq 155, length 40
10:38:55.927586 Out ethertype IPv4 (0x0800), length 76: 172.245.215.48 > 46.246.9.143: ICMP echo reply, id 1, seq 156, length 40

Check that server sends ping echo replies:

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$ tcpdump -nni venet0 -e icmp[icmptype] == 8
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on venet0, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
10:39:37.619442 In ethertype IPv4 (0x0800), length 76: 46.246.9.143 > 172.245.215.48: ICMP echo request, id 1, seq 162, length 40
10:39:38.615616 In ethertype IPv4 (0x0800), length 76: 46.246.9.143 > 172.245.215.48: ICMP echo request, id 1, seq 163, length 40
10:39:39.623875 In ethertype IPv4 (0x0800), length 76: 46.246.9.143 > 172.245.215.48: ICMP echo request, id 1, seq 164, length 40

Run tcpdump and try to ping your server again.