Well the all of a sudden piece will because you will not be expecting it.
The cause may be due to an interruption in the external power - a bad connection - but it's more likely to be a heat-related fault in the freeview box. A failing component or perhaps a bad joint.
As the box warms up the fault comes on at a certain temperature - then it begins to cool down and the fault goes away and it begins to work for a while.
In a previous life I used to observe faults like this and we used to use a can of Sevisol freezer spray (Amazon £11) to selectively cool parts of the equipment to discover the failing piece.
Heat faults are often - but not exclusively - in the internal dc power supply of the equipment.
Have a go at repairing it - have someone else do it for you - or buy a new box.