If there's no specific error or message, this could be a browser issue. You'll want to use an incognito window and create or edit an invoice from there.
Google Chrome: Ctrl + Shift + N
Safari 11 or newer: ⌘ + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
Also, clearing the cache can fix errors caused by these junk files. Lastly, make sure that the browser is up-to-date.