I was on NSL just now and saw it happened too.
I was on Yishun - Woodlands direction when half-way at Sembawang when the driver told us to alight the train at Sembawang due to technical problem. The train stopped and everybody alighted then the train moved backwards (back to Yishun again) Suspected that this train went to aid the Yishun - Marina Bay way
Boarded the second train and saw a half-way stopped MRT paused there. SCDF people cordoned off the area. I wondered what happened there
When I reached Woodlands, people were at the Control Centre asking what happened and there was a chaos there. The staff had difficulty to calm them down. There was a board saying something about it but I am blocked by the angered commuters.
As expected, many just went to Causeway Point to waste the time
1h later (I am not sure), its back on service