I have been using DT Register very successfully until the Calendar suddenly stopped functioning. When I click on the link the calendar page appears but the calendar is blank.
Well, DT Register features will never just stop working on their own. SOMETHING has been changed within your site or your server that has caused the issue. First step though would be to update DT Register as 2.7.3 is old and not supported. After updating, if the problem remains, you need to track down what changed in your site/server. Something did.