Let me get the help you need to ensure that you'd able to access Customer window in QuickBooks Online.
I have checked this issue here in our system and we have received no reports from other users getting the same challenges.
To get this be fix, I recommend accessing your account in a Private browsing window first to verify if this is caused by the system or your browser.
Here are the keyboard shortcuts that will help you accomplish this in all supported browsers:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift +N
Once able to open a private browser, kindly access your account again then check if you're now able to access Customer window. If it works, then your main browser may have too much historical data from previously visited websites which slows down internal processes and causes this kind of behavior.