I have found that with ADSL I fairly often used to get dropped connections. The frequency varied - I had a string of them over a period, then nothing for weeks. It seemed to be happening at about lunch time.
I assume that the telephone line had a voltage blip. I didn't notice anything on the phone - chances are I wasn't on the phone at the time - but it did disrupt the ADSL service.
Strangely enough, I still had the modem reporting that I was connected to ADSL with full line speed, but no response from the internet. I had to disconnect and reconnect to re-establish my internet connection.
I am a fair distance from the exchange (initially I couldn't be offered 1 Mbit) so that might affect the frequency of the problem.