Indigo Airlines — boarding issue

Sir
I had a flight to board from new delhi to udaipur on 29 july 2017 from flight 6e746
Scheduled at 7.40 am
I checked into the boarding at 6.55am and did my security checkin.. I got a call around 7.11 due to technical issues i coundnt attend the call.. By 7.23 i went down at the boarding gate and they didnt let me enter.. The flight was supposed to be at 7.40
I missed my exam because of them..
They should have let me in and i want a refund back