I had a similar occurence yesterday - No internet at all for the first time in many many years - ntl256 modem had sync light flashing and ready light flashing now and then - faults tried a reset with no success then he had a less than convincing check to see if there was any area faults (none apparently) before booking an engineer for today, he then said try rebooting the modem every couple of hours and if it works call back and cancel the engineer
This morning and all day the connection has worked faultlessly (without resetting) however by some strange coincidence all 6 of our mailbox addresses had their passwords reset overnight ! - when I called faults again this afternoon they said they have to do this from time to time due to SMTP corruption!
All sounds a bit bizarre really !