This is not a problem at all. There is so little information in this file compared to anything else, that you will re-created it very quickly.
Simply create an empty solution with Visual Studio. Check up if the file is placed in right directory. The perfect location of the SLN is where all the project files (*.*proj, for example, *.csproj) you need to have under this solution will go directory structure below the directory where your SLN file is placed. If you saved the new SLN file in a wrong place (which is very easy to misplace), don't save it again. It's easier to exit Visual Studio and move the SLN file in proper location manually. After that, click on this file to load Visual Studio again.
You solution file is created and placed in a right place. Now you need to add all project files to it. You may want to refine your solution structure down the road using "Solution Explorer" => you solution node => "Add" => "New Solution Folder". The solution folders will not alter the directory structure of your projects and won't affect build or run-time of your projects. Add the projects one by one using "Solution Explorer" => you solution node => "Add" => "Existing Project".
Final step: check up project dependencies via Main Menu => Project => "Project Dependencies…". This step is not needed is your projects referenced each other using project references (recommended), when the references were added using "Add reference" => "Projects". If you made the references this way, the project dependencies will be set automatically based on referencing relationships, which are recorded in projects, not in the solution file.
That's it. All other information on project structure, options, etc. is kept in project files *.*proj. Hope you did not loose them. :-)
Main recommendation: stop doing any development without Revision Control System! Those systems are light-weight, free of charge and easy to use.
Don't risk your code asserts.
See the following discussion:
Revision control systems, which to choose from?[
^].
—SA