Hello and Welcome to Windows Central,
I have experienced this but only for a few moments - it's possible that the servers are currently busy with the release of the Windows 10 to users worldwide. You may try any of the following:
- wait a little while, e.g. around 30 minutes, before trying again
- restart the Store App
- restart your PC
Should you wish to provide us with more details or ask further questions, please don't hesitate to create a Windows Central account and respond to this thread.