After logging in to your Sandbox account, it is possible that you are not able to view your dashboard. This can be because of a cache issue. It is recommended that you clear them, restart your browser and access your account again. 

If it does not solve the problem, try to access it using a different browser. If the issue still persists, contact support.

Additional Resources 

How to clear Cache from Chrome

How to clear Cache from Safari

How to clear Cache from Firefox