I sign in then get a blank page can anyone tell me what's gone wrong?

There are instances that the unexpected issues with QuickBooks that aren't working, can be resolved by performing some basic troubleshooting steps, Harleymovin.    

Let's start accessing your QuickBooks using a private or incognito window. This helps your browser to load faster as it doesn't store any file history. The stored data in your browser may result in unexpected behaviors.

Here are the keyboard shortcuts to operate in such mode:  

• Google Chrome: Ctrl + Shift + N
• Internet Explorer: Ctrl + Shift + P
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Option + P

Once logged in, access the data again to double-check. If it works, return to your default browser and perform a clear the cache and cookies. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system, and you'll be able to work with a clean slate. If the issue persists, try using other supported browsers. 

I've got an article to learn more about fixing unexpected browser-related behaviors: QuickBooks Online doesn't load correctly or has blank pages. 

Feel free to get back to me if you have follow-up questions. I'll be around. Have a great day. Take care!