Did you change the name of the user after their buddy list was setup? If so all contacts would have been lost see:
Bug 27877 - Renaming account breaks Instant Messenger IM
As for Pidgin settings here is an exact example:
Email address is :
user@domain.tld
server accessible by: email.domain.tld
Pidgin account setup:
Basic tab:
Protocol: XMPP
Username: user
Domain: domain.tld
Resource: whatever you want. Only one resource name can be connected at the same time.
Local Alias: useful to set to what your name appears as to other people since without it your name in chats will appear as
user@domain.tld
Advanced Tab:
Connect server: email.domain.tld
If you want the connection to be encrypted the check require SSL/TLS