Odds are the account is still there. My guess is possibly it was made "Inactive" and you can't see it anymore.  Either that or there is a Filter on the list you're looking at and again, it's just now showing up.

First check would be to go to you Chart of Accounts and then go to the Settings (little Gear icon) and check the Include Inactive Accts.  If your account shows up, you just need to modify it and uncheck the Make Inactive so it becomes Active again.

Another way to check to see that the account is really still there and just hidden for some reason (maybe there is a new BUG with QBO) is to go to one of the last Vendors you just paid out of that Acct and look at the transaction.  In theory you should be able to pull up the payment vs getting some sort of error if QBO really can't find it.  Also maybe run a Balance Sheet Report and see if the account shows up. 

But the first "is it plugged in" check I'd do is to make sure the Acct didn't just get switched to Inactive. 

Let us know what you find or if you're getting any errors.