can connet to the internet, but CANNOT response ping from internet...

Hi everyone,

I used to share a DSL line among my computers in my home network
using a router. But last week I decided to give my FTP server (Fedora
3) a dedicated bandwidth and therefore I subscribed another DSL line. I
have setup my Linux box to connect directly to the internet using
Roaring Penguin PPPoE client. I found that my Linux box doesn't
response to ping from internet. (I can ping myself using internet IP
address though)......Now My FTP server cannot be accessed from internet
and it really causes me troubles as I need to use it very frequently.
Does enyone know what happened? I would be greatly appericated if
anyone can help me get my FTP up again.

P.S: I am a linux novice and I dont know much about setting routing
rules, iptables/netfilter and related stuff. But I think my problem is
related to that.

Thank you for your time reading my post.