Tried downgrading Nexus 7 from Android 5.1.1 to 4.4.4, and it is bricked

I rooted my Nexus 7, and it was working fine on Android 5.1.1 - albeit very very slowly. I got tired of the lag and tried to downgrade to Android 4.4.4 following this link . The tablet went into bootloader (fastboot) mode and stayed there for a long time. At this time, I could cycle through START, RESTART BOOTLOADER, POWER OFF, and RECOVERY MODE options, but NONE of them work except OFF. So I switched it off, and now, it does not boot up at all, not even in the recovery mode. It is just blank, I see no sign of life at all. I cannot get it to start, so there is no way I can use ADB or Fastboot for this. I already have adb/fastboot setup on my PC, but it does not recognize the tablet. I did go through the similar question https://android.stackexchange.com/questions/126570/i-have-a-hard-bricked-nexus-7 but it did not help. Is there any way I can solve this?