Can anyone advise: All of a sudden my bank account is not linking to my quickbooks account. It won't refresh. Suggestions?

It could be due to a couple of issues. 

First, try logging directly into your banks Online website.  Besides confirming they're not down for some reason, also check to see if they have changed their security protocols.  They may require a text or have made some other change.  If it just happened, it may take a few minutes for QBs to change things on their end.  Also if you see changes, reach out to your bank to see if they already know of a fix. 

The other most common thing is the links just get messed up in QBs.  Go in to your Bank Account settings and DISABLE the online connection.  Then try reconnecting it and see if that fixes it. 

NOTE: Sometimes when you reconnect you'll find QBs downloading duplicate entries!  So before you try syncing with the bank, just take a note of  your balance at the last point you Reconciled and any outstanding items at that point.  If you do get duplicates, the ones prior to your last Reconciliation should be easy to find and delete.  Your balance on that date should match your notes. 

The more current duplicates should also be easy to find.  But even if you miss any, they will show up when you next Reconcile the account.  ALSO, remember the date from the download is the date the transaction cleared the bank.  So any checks you wrote may be dated on the 5th, but may not show up until the 25th.  Just keep that in mind as you're looking for duplicates.