DNS Lookups Failing Suddenly - Please Help

All of a sudden, DNS lookups don't work. When I went to bed last night, everything was fine. This morning, after doing nothing to the server, resolving an IP address to a hostname makes an attempt for 10 seconds then times out (IP masked for forum):

Code: 
 dig @213.208.***.***

; <<>> DiG 9.2.4 <<>> @213.208.***.***
;; global options:  printcmd
;; connection timed out; no servers could be reached
It's not just my IP but all IPs. I found out because my site, running on my server, uses the gethostbyaddr PHP command and every page was taking 10 seconds to load as a result.

I have two DNS servers, both running on the same box. Bear in mind - I've been running this configuration perfectly and without a problem for 6 months.

I must stress, I've changed nothing. If someone can help me I'll be eternally grateful. Many thanks.

 

 

 

 

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