If you disconnected an account, you can see this account greyed out at the end of your account list  in the group 'Disconnected'. 

To see this account again, please proceed as follows: 
'Menu' > 'Settings' > 'select credentials' > 'Connected Accounts' > 'select the disconnected account' > 'Settings' > 'remove checkmark from 'Disconnect Account'

Or you can proceed like this:

'Select the disconnected account at the end of your account list' > 'click on the dropdwon besides the account name' > 'Settings' > 'remove checkmark from 'Disconnect Account'

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

If you hided an account, it will no longer be shwon in the list of your accounts. 

To see this account again, please proceed as follows:
'Menu' > 'Settings' > 'select credentials' > 'Connected Accounts' > 'select the hidden account' > 'Settings' > 'remove checkmark from 'Hide this Account'