Has the maintenance expired, or does it only expire today? I seem to remember you'll get a message saying that maintenance wasn't performed due to lack of cash. You certainly get one when marketing campaigns are stopped, or when an automatic lease extension fails.
If you go to the aircraft's page and see 'C check due in 0 days', then it should get done when the day changes. If it's beyond that, it should say C-check expired, and you should get a big warning message on the dashboard page.