Mac or Windows, and EXACTLY which version of the operating system?
Recent Mac AND Windows operating systems have been known to cause "odd" problems
And, which version of Lightroom?
Have you tried Launching Lightroom and holding Ctrl+Alt during this Launch?
Usually Launch issues for Lightroom are either Activation or Catalog problems.
Most often Catalog, so trying with a New Catalog you can determine whether the problem is coming from Catalog, or from Activation.
Hope this helps.