User Missing on Login Screen but still in Users & Groups

MacBook Pro early 2011 with Sierra OS 10.12.6 ran fine with Primary User 1 and Secondary User 2. Most work was done in Mail with all Mail sent and received saved to On My Mac. Login Window to Name and Password allowed login to each User. Then On My Mac disappeared for User 1 in Sidebar and about same time Login Window stopped allowing access to User 1. Filevault is not being used. I created Administrator privileges for both User 1 and User 2 and can back them up by drag and drop to another Hard Drive. Trying Login Window access to User 1 goes blue screen, after a delay starts a Menu Bar and after several minutes comes back to Login Window. Trying Login Screen to List of Users stops cold, apparently because it can't provide a List of Users that includes the missing User 1 so I have to do hard Shut Down with Power button. I can't recall how I got it to provide Login Window to Name and Password again. For days I've been trying to provide access to User 1 and its On My Mac mailboxes where many mailboxes are used to keep track of Mail sent and received. I'm falling behind handling mail since I can't access prior correspondence that is in On My Mac for missing User 1. Can I switch Users without doing new Login? All suggestions would be appreciated.