You would treat it in a similar manner to an engine failure after take-off, depending on the altitude obtained would determine your choice of landing area/field.
If by the top of the winch launch and cable could not be released from the cockpit then there are two fail safes, once the glider has flown past the vertical point of the cable then the tow cable will release aft, if that doesn't work then there is a weak link in the cable system that would break once the downward pull force is exceeded.