Trip Computer resets after 2 hours

2000 Passat 1.8T. Since it's last service, trip computer resets after car is switched off for 2 hours. Previously, it only reset when I wanted it to. Are there 2 modes that I can switch the trip computer between? If so, how? I can't spot it in the manual.
Thanks.

Found it. I could have sworn I'd tried the obvious before (ie the "reset" button!!). Anyway, it's worked now and my little nagging concern is fixed. Thanks.