That sounds rather odd. Please try the DotNet repair toll from Microsoft, maybe it helps with diagnosing the problem:
https://www.microsoft.com/en-us/download/details.aspx?id=30135
If you are using Windows 10, you can also try to reinstall the DotNet Framework 3.5 from the Windows control panel.
a) Open the “Programs and Features” control panel
b) In the “Programs and Features” window, click on the link “Turn Windows features on or off”.
c) Check if the “.NET Framework 3.5 (includes .NET 2.0 and 3.0)” option is available in it.
d) If yes, then enable it and then click on “OK”.
e) Follow the on-screen instructions to complete the installation and restart the computer, if prompted.