Your MyMonero wallet may be “catching up”. When logging into MyMonero on a new device, you will have to wait for it to load all of your transaction and block data that is normally cached on your regular browser. If the problem persists after a few minutes, try a different browser. MyMonero requires a modern browser that supports current and secure cryptography, so please ensure your browser is up to date.
If this issue does not resolve on its own, please contact the support team for further assistance.