The Convert Lead button has suddenly stopped working in Classic. The button is on the lead page but when clicked nothing happens. No changes have been made to permissions and I've rechecked them all. If I change to Lightning the Convert Button works. And it works in the Sandbox for Classic.
There are no error messages.
I've been working on this instance of Salesforce for a couple of months and never had this problem. It only became a problem today.
UPDATE: The Find Duplicate button has stopped working as well. Nothing happens when I click on it.