The tunnel was closed after a truck damaged cables connected to fire detection equipment.

The cables first needed to be replaced before traffic could once again be allowed through the tunnel named after Field Marshal Montgomery of Alamein, an Allied hero of the Second World War.
The accident with the lorry happened on Tuesday. The ventilation was damaged too. The tunnel had to be closed while repair work was completed.
The repair work proved to be a complicated undertaking with the deadline for the reopening changing throughout Wednesday.
Inge Paemen of Mobile Brussels: "The cables had to be replaced over a 450 metre long stretch of tunnel."
Fortunately the work was ready in time for Thursday morning’s rush hour in the Belgian and Flemish capital.
[Flandersnews.be / Expatica]