How do I add an exception a website?

I used to be able to access my banking site, but now I can’t

First, I’m an Avast user and not an Avast Team member.

Are you getting any error when trying to connect, if so what ?
A screenshot may help.

As you posted in the General sub-forum there is no clue as to what Avast Program/s you are using ?

Are you using a VPN as banks, etc. can be somewhat picky about people hiding their location ?

You could try adding an exception - Avast UI > Menu > Settings > Exception,
See an example image, the * is a wildcard for instance to save entering a sub-domain or www, and any file/files in that range, etc. car has to be taken not to have a loose exception as it could leave weaknesses.