If your GTM account is messed up with so many accounts it may be possible that you have deleted a different account mistakenly. It can also be possible that you see a GTM account missing. So there may be two possible cases
Your access has been revoked
GTM account was deleted
If your access has been revoked you can ask your client to give access to the specific account.
In case if your GTM account has been deleted it can not be restored and you will have to create a new container and redo all the things again.
see the original question here: https://productforums.google.com/forum/#!topic/tag-manager/iJ0RbFi5fEQ;context-place=forum/tag-manager