HZN never went much below $4.50 after 9:45am. Also, if you placed buy market order then it should go through even at higher price like $4.60 or $4.70. So I suspect that your order was not a market order. If it really was, then you may want to contact your broker. Which broker do you use, btw?
Only 2 possible explanations imo 1) You thought you placed the order but actually did not (it's happened to all of us) 2) You thought you placed market buy order but place a limit order by mistake Show us a screen shot of the order status, just black out any personal account info