If there aren't any signs of life by now (or by May 15th, really), then they are unfortunately deceased. Where are they planted/what is the climate like? Hops are basically weeds and pretty hardy, so a 50% survival rate is puzzling to me. Was your winter overly brutal? If they are potted, then there is a higher risk of death because the cold will soak through the pot and it will get colder than the ground.
Even if you can get rhizomes at this time of year, I would not plant anything this year (unless you are in a very mild climate, like California or Florida). My general rule in Illinois is that if it isn't in the ground and growing by the first week of June, it is getting pushed to next year.
Also, with a 50% survival rate, I would order from a different supplier (or at least contact them about it - they may give you a discount on the next set). You may want to go to a different variety if your climate is on the harsher side - Cascade is very hardy, for example. Also, 20 plants worth of the same hop is going to get boring quick. Personally, for 20, I would do 4 amarillo, 4 citra, 6 cascade or centenial, and then a couple of haullertau or saaz. I would avoid "basic" bittering hops like Magnum or CTZ - using pellets for those is a better use of time and money (also, a 3oz charge of bittering leaf hop is a LOT of volume).
Anyway, my 2 cents and ramblings. Better luck next year.
