Why does my Prima F sometimes run all night when timer has clicked off?

When the timer switch on my control panel clicks off at night, sometimes my Prima F keeps on running and firing until the morning. The pump also keeps running but there is no hot water circulating round the radiators. Once it is in this mode the only way that seems possible to break the cycle is to switch the whole system off for a couple of days and then restart it. Can anyone help me understand what may be going on?