After upgrading to 4.6.18, cannot enter credit card details in event registration
I had a look and can you entire billing block is indeed MIA.

Two thoughts: 

1) get into the Event Registration Config and try re-adding the Payment Processor - re-save

2) check for errors in your ConfigAndLog/ when you load this form

EDIT - more thoughts:

3) check the Payment Processors their config - and create a (Test) Contribution page where you run a transaction with a fake credit card

4) (in general) check the Manage Extensions screen and look if there are any upgrades for your Payment Processor (of course this only applies to those which are not baked in core)