Applied for OPT dec 14, got denied APRIL 28 15.program end date on I20 JAN 11 15.When does my grace period start?

Hi All, I applied for my OPT in dec 2014 and it got denied. The program end date on my I20 is JAN 11 2015. They took really long time to take decision on my OPT and the denial notice was given on APRIL 28 2015. When does my grace period start from in this case? Is it from the end date of program or the date on which I got decision on my pending application? Also my sevis is in COMPLETED state. what are my options to remain in status and continue to stay in US until I get my pending H1B decision?