Unknown type or namespace "Sitefinity" of "Te

The test site for learning was working great.
Then I was fighting with adding a blank test control.  I am new at working with web apps, so I selected rebuild, then sitefinity lost the reference.  I tried build again but got errors.  Build now fails, and the site want come up.  Luckily for me I did not use this on a live site yet.

What did I do wrong?

My setup I have a the  last version of licences Telerik Ajax ASP control, Just Code, Telerik Silver light installed on my computer before I set up sitefinity 4.0..  Wondering if I have a conflict of some type since I had all those before trying 4.0.  I know in 3.7, telerik was always wanting to update the bin, but always say no.   If I accidently over wrote a dll, I would copy from a back up.

How do I restore back, without starting all over, I am sure I will run into this again.

Thanks
David 

I got the problem resolved by restoring back to a point before HP decided I needed some updates.  This time I did not allow HP update the computer. 

I created a community edition barebones project for backup files this time.  When I open the Barebones by selecting Edit in Visual Studios I got  a pop up informing my the project uses a trial edition of Telerik tools in this project. It ask me If I would like to install the dev version.  

I do have a current liciense with the most current version of tools, should I update.  BareBones, and a new site I am setting up for a local ham club are both community editions. 

Think I will hold off updating a day job site until I figure out what happen here.

Thanks David