Not sure how exactly the process goes with stocks being halted, but it happens when big news like mergers, bk's etc are coming out during trading hours. I had it happen when I was a newbie one time on DIS while I was in a position. They were scheduled to report earnings after the close, and news leaked prior to market close. the stock was halted for 15 mins. or so until the company made an unscheduled early release of earnings. You can cancel your pending orders and put in new orders, but nothing is filled until trading resumes, typically at prices far from where stock was previously trading.
This is obviously not an everyday occurence, so no point in stressing about it, but I think Kings point is that there is a lot of news surrounding C and the banking sector in general right now, so trading it can be very treacherous for a new trader.
BTW, I used to trade C along with JPM and BAC quite a bit. Not so much intraday anymore. You see the volume, obviously these are among the most heavily traded nyse stocks, so somebody is trading them. I just think if you are trying to learn, maybe picking something that has some volatility, but not so much news, might be a better way to go. 'nuff said. Anyway, again, good luck with your trading.