If an account is disconnected, it's shown grey at the end of your list of accounts in the group 'Disconnected'.

Please proceed as follows, to see the disconnected account again: 

‘Accounts’ > ‘select disconnected account’ > ‘click on the dropdown besides the account name’ > ‘Disconnect this account’ > ‘OFF’

The account is now displayed as asctive again in your account list and the group, it belonged to.

If you hided an account, it is no longer visable in the list of accounts. 

Please proceed as follows, to see the hidden account again: 

‘Settings’ > ‘select credential’ > ‘select the hidden account’ > ‘Hide this account’ > ‘OFF’