I don't know the particulars about your bank, obviously but the thing is, mainframes serve so many millions of clients, and that's clients of the largest money transfer networks (Amex, Visa, Master, everyone really) that if there's an outage it will make the news, internationally, and it will be front page news too. With live updates.
In fact, if I think about it, I don't think I remember any time when a serious outage that was eventually explained in the press was the fault of some mainframe going down. Usually it's something else, like someone misconfigured something or something didn't update correctly etc. stuff that sounds a lot like day-to-day web dev stuff.
So I think maybe it was something else that went on with your bank, that only affected your bank. Like inkyoto says below, maybe some DNS went down?
In fact, if I think about it, I don't think I remember any time when a serious outage that was eventually explained in the press was the fault of some mainframe going down. Usually it's something else, like someone misconfigured something or something didn't update correctly etc. stuff that sounds a lot like day-to-day web dev stuff.
So I think maybe it was something else that went on with your bank, that only affected your bank. Like inkyoto says below, maybe some DNS went down?