I have previously installed 2014, and had no problems connecting to the default server (SQLEXPRESS). Now, a month or two later, I am diving into it again, and I opened up the Management Studio, was able to add another database - going through a tutorial, and then tried to right click on it - but got an error about a single user server, and something about admin. So, I tried to switch account login to sa, but I must not have created one. Then I switched to a local account, but that didn't work, so I tried to go back to the default, and it doesn't work.
I have not tried uninstalling the software, yet, but if I remember right, it took me a while to download it all. I also have Visual Studio installed so I could learn C# and use that with SQL Server.
I've googled this login problem, but I haven't figured out an answer yet.