As you are abroad, your roaming charges may have reached a level that O2 are not entirely comfortable with. This would be your unbilled usage, which normally wouldn't result in a ban, but if these charges go very high O2 would bar the phone until you make an interim payment to try to ensure the bill doesn't go out of control. Have you checked your unbilled usage? It might be wise to call cust services for free on +44 844 809 0200 (from any country) and ask. I wouldn't bother with online chat for a billing issue.
Track usage, upgrade and get support with the My O2 appDownload My O2