My systems wont connect!!!

Hello,
i need help, l have a complet network in a location and another complete network in another location, i try accessing this two network from each other but its not accessing.

the complete network includes a Domain server, ISA server and computer machines as a branch office and the same gadget for the head office too. i can have access to the internet from the two ends but cant have access to the resources in head office from the branch office. Please i neeed urgent help.

Regards