Active Directory Connection Problem : Urgent

Hello everbody,

I am new to this forum and also new to networks.
I have a problem connecting from other machine to Active Directory. Here are the details

1) Active Directory on DNS Machine on Windows Server 2003 OS
2) Red Hat Linux V5 Machine with Websphere Server
3) Red Hat Linux V5 Machine with DB2
4) Red Hat Linux V5 Machine with LDAP
5) My host machine with Windows XP

My problem is :
1) I am able to ping from Active Directory to other machines but am not able to ping Acitve directory from other machines..
2) From my machine(windows xp) , i am not able to ping Active Directory nor from Active Directory can ping my machine

3) From the active directory, now i am even not able to access the internet .
Initially all was working correctly but now nothing seems to work today.

can anyone help me out of this .

thanks in advance

Mayur