Help, My Nexus 7 is dead!

I have a Nexus 7 2012 version since 2014, after many updates and installing apps it was becoming slowly and unusable, so I dicided to flash and install Cyanogenmod 12.1, after installing it, all was fast and smooth, I loved it, but after a month it was turning laggy again, so I decided to use Nexus Root Toolkit to come back to 4.4.4 stock. After flashing, the tablet still in recovery mode, I selected START using the buttons, and the Google logo appeared but a little message in the corner of the screen scared me, it said "booting failed" , so I tried to flash again, but now using the option "the tablet is bricked/bootloop" but it didn't work, so I tried to turn off the tablet and power in again, but nothing happened when I tried to turn on again, the tablet didn't turned on and now it still off, when I plug the charger the battery logo doesn't appear and tried to press the power button for 30 seconds without success, I need help guys, someone told me that it will be possible to fix it, but he doesn't know how to do it.
Thanks for your attention!