This situation can be caused by numerous items. A search on Google will provide more results than you can likely handle. However, maybe these tips will help you get to the bottom of the issue.
- Disable your Extensions, one by one (so you can find the culprit)
- Restart Visual Studio and see if that resolves the issue.
- Do until all extensions are disabled.
- Productivity Tools has shown up on the radar of many troubles - so try that one first.
- Restart your VS environment.
- Close out of any instances.
- Ensure all IE instances are closed.
- Ensure the Task Manager doesn't show any instances for devenv.exe, IExplore.exe.
- If they do - end them.