For a lot of people it has to do with plugin conflicts (as it did for me). You might explore that if you have any security plugins or SSL plugins.
Otherwise, it would be worth playing around with WooCommerce's SSL settings on the General tab of the Settings page. Disable "Force SSL" or "Un-force SSL" for a moment to see if it fixes the problem.
Generally of course you WOULD want to force SSL to be on for checkout, because otherwise people might be sending you credit card info in an unsecure way.