OK we have found the issue.
Occasionally we get a dud coupon that goes bad and jams up the Wordpress site that Coupons runs off of. That site then goes into lockup and pulls tons of server resources. That locks up the whole box that all our Wordpress ancillary sites run off. This is the problem with mickey mouse apps like WP but there's little we can do about it other than catch it once it's playing up.
The coders are on it now. Thanks for flagging it up. The site manager SmokeyJoe is in Aussie land for a bit so things sometimes get missed for a time, it should be better when he gets back.