can't open query monitor from toolbar

What shows up in the toolbar? Does it say “Query Monitor” or does it show the page load time, query times, etc?

Can you check PHP’s error log and see if there are any errors? if not, can you manually view the source code of the page and see if there’s a fatal error at the bottom?

Can you revert to the default theme and see if the problem goes away? If not, can you disable your plugins one by one until the problem goes away? You might have a plugin or theme on your site which is doing something crappy with scripts or styles.