removed as member of a group where I was the admin after reinstalling whatsapp after it crashed! I was the only admin in the group that I made, now after the app crashed and I reinstalled it - It says "Error you are not a participant in this group"

So that means the group exists without any admin? How do I reinstate myself as admin and get the group back? Do I have to make it all over again &amp; lose all the old messages we had going over all this period of time?
Now there can be more than one admin for a WhatsApp group. If you choose to delete the group (where you are the sole admin) and remove yourself, one of the member in the group is selected in random and that member will become the new admin. 

In this case, somehow your account got deactivated. Hence you were removed from the group. Someone else in the group will be the admin now.  You can ask the current admin to add you back and make you admin.