What happens if a survival curve doesn't reach 0.5? Does this mean you can't compute the median?
 A: Yes, unless you use a parametric approach and are willing to extrapolate.
See SAS Lifereg.
A: The median survival time is defined to be the time at which the survival curve crosses 50% survival. If the curve doesn't cross 50% (because survival is greater than 50% at the last time point), then median survival is simply undefined. More precisely, it is greater than the last time point on your survival curve. The only way around this, as B_Miner posted,is if you fit some kind of model so are willing to extrapolate beyond the time span you have data for. 
A: "The median is the data point which is greater than 50% of all data points. As soon as you have one or more data points, you always have a median. What does the phrase "doesn't reach 0.5" mean? Please clarify. "
In a survival curve situation, you have N individuals followed over time and let's say only 10% of them die in 5 years. Until 50% of them die and contribute a survival duration, you cannot measure the median survival of the whole group of N people.  If the question were, "What is the median age at death in your town?" then you could look at the people who died and calculate median age at death. This is a cross-sectional analysis of those who died. But consider instead of a cross sectional study, you do a cohort study. If the question were "What is the median age at death among people born in 1998?" then you can only provide age at death among those who died, but not among the whole cohort. We can break down the latter question into 2 different ones and they yield 2 different measures. (1) "What is the median age at death among people born in 1998 who had died by Sept 7, 2013?" You can answer this question if you have any number of observations (deaths) greater than 0.  The question in issues of median survival in a cohort study is more often this: (2) "What is the median age at death among all of the people born in 1998?" For this, you need 50% of people to have died to be able to measure the median survival. If not, the survival curve, which starts at 1 (100% alive) and ends eventually at 0 (100% died), does not cross 0.5 (the point at which 50% have died).
