Targeting Pack for this framework version

Hello,

I am using Sharp Develop 5. I recenly tried to create a project when this error message poped up:

"The reference assemblies for framework ".NETFramework,Version=v4.0" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly may not be correctly targeted for the framework you intend."

Any hint on how I can solve this would be very much appreciated? It was initially working without problems.

Best regards,

Senior Owams.