Partitions wiped

Hi, i need help. I have rooted my tablet with TWRP to use a custom rom like many of you guys and it seems like all the partitions have been wiped.
When i try a stock rom installation, it says E: can't mount data or system and then my tablet freeze. It there someone who have a backup of the partitions for a nexus 7 16gb? Or someone who has had this issue before and willing to help?

Thank you in advance