The best way to check Google Public DNS for problems with a domain is to go to the dns.google.com diagnostic page and type the domain name in.
When I did that for tillerhq.com, I initially got the error "All name servers for zone unresponsive (no query sent)." I tried flushing the domain at https://developers.google.com/speed/public-dns/cache and now I am getting this error: "Name servers did not respond [18.104.22.168, 22.214.171.124]."
If TUCOWS / Hover DNS operations were being hit with some kind of denial-of-service attack, it might be the case that they are blocking our outbound resolver IP addresses as listed at https://developers.google.com/speed/public-dns/faq#locations. Alternately, if the problem was network connectivity, it is possible that their connectivity is back up in general, but peering with Google is not.
All I can suggest is that you contact TUCOWS / Hover and ask them if they are seeing queries from our outbound resolver IP addresses generally, and whether they are blocking them in any way.
If they are not seeing any queries from us then it is probably a routing issue between them and AS15169 (Google's main network number).
If they are seeing queries from us, but are blocking or not responding to them, we won't be able to resolve their domains until they do fix that.