what did you try yet?
Do you have access via SSH?
Can you ping the machine?
Did you try accessing via port 8006? Via HTTPS?
What's your network situation? What the network configuration of your client? PMG itself?
Etc. etc.
There are a lot of causes, without any information it hard to help you
What is the exact output from curl -v https://<your-pmg-ip>:8006?
Again, without providing any configuration/logs/errors/etc. it is just a guessing game.
What is your network configuration, both client and PMG (cat /etc/network/interfaces)?
Do you have a firewall configured between those devices? Etc.