Not able to send or receive emails: MX suddenly not configured properly, was fine before!

Not able to send or receive emails: MX suddenly not configured properly, was fine before!

Help! My emails stopped working at around 8am this morning, and when I try to send any test emails to the account is says gives me an undelivered reply with the following (my domain changed to 'mydomain.com');

DNS Error: 41662254 DNS type 'mx' lookup of mydomain.com responded with code NXDOMAIN Domain name not found: mydomain.com

It's been fine for years, why suddenly not working?

I have the following mx settings in my hosts settings (Fasthosts)
mx.zoho.com - 10
mx3.zoho.com - 20

Thanks