Before you left to go abroad did you check that roaming was active on your account?
(If pay as you go it should be on by default but if a pay monthly customer, you will need to have been with Virgin Mobile for more than 3 months to activate roaming).
Log into your Virgin Mobile account then click on the "Plan and device" tab.
Go to "Manage my services" > "Roaming" needs to be set to ON & "International call barring" needs to be set to OFF
If they are not already, make the change by moving the "switch" in your account.(Click the other side of the rectangle box).
If you have to make a change to the settings, allow time for your account to update then reboot your phone.(Virgin advise that this may take upto 24 or longer from abroad).
I would periodically turn the phone off/on to see if it finds and selects a network.
You may have to go into the phone's settings and do a manual search for network.
Also check that roaming is enabled on the handset itself.
Here is a useful help page from Virgin: Taking your mobile abroad
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