Using +New or New transaction, neither opens up window to write a check or create an invoice.

A pleasant day to you, @naturf.

Let's get this issue sorted out so you can create transactions and get back to business.

Being unable to create transactions due to unresponsive windows in QuickBooks Online can be caused by the stored data files on your browser. Web browsers gather cache files to speed up your browsing. However, too much can cause unusual behaviors like the one that you've experienced.

To fix this, I recommend accessing your account using a private window. It opens your account in a clean slate and loads fresh data.

If you can create transactions from there, you can go back to your regular browser and clear its cache. Please see this link for more details about this process: Delete or Disable Cache and Temporary Internet Files in Your Web Browser.

You may also consider checking out this article to guide you in managing transactions in QuickBooks Online: Getting started with QuickBooks Online.

If there's anything else that I can assist you with, let me know. I'll be right here to help you. Take care!