Hero Motocorp — engine seized in 33924 km

Address:

New Delhi, Delhi, 110045

I have procured hero i-smart with registration number dl9sba 3442 in july 2015. I have gone through all free services. One free service was not considered by the service centre "shiv service station, sec-1, dwarka, new delhi" as the it has exceeded the km limit for 25 km. After all paid services were carried out.
In my last paid service which was done on 6th may 2018 for 33.5 km. There was no issue in the bike prior to the service. Neither the service centre pointed out any issues in the bike. After 2- 3 days there was some sound coming from the engine side. On 21st may the engine was seized as per the service centre people.

Need to know when there was no issue then how engine can seize. I had to pay rs. 11555 for the repair.