Cause
There are two likely causes:
that you did not sign out from Practice Management before you shut down your machine
That your machine crashed when you last used Practice Management.
Both can leave an entry in the Digita database connection table for your user account, so that Practice Management has a record that you are still signed in.
In this case, you will not be able to sign in and the connection is counted as a user licence, so your organisation will not be able to use its full number of user licences. The database connection should automatically end after 20 minutes, which will resolve the error.
If the connection does not end, ask your organisation's IT professional to take the following steps to download and run an SQL script to clear any stale connections.