unblock

verb

1. (transitive)To remove or clear a block or obstruction from.

   “We need a plumber to unblock the sink.”

   “During this period, Network Rail was able to unblock a culvert and stabilise a high-risk landslip at Beattock, and to stabilise a high-risk landslip site at Ravenstruther (near Carstairs).”

2. (transitive)To free or make available.

   “It is not just bad news about Dubai that gets blocked from the Internet; […] a whole range of other social networking sites have been blocked, unblocked and reblocked.”

3. (intransitive)In whist, to throw away a high card so as not to interrupt one's partner's long suit.
4. (transitive)To enable communication again, via telephone, instant messaging, etc., with (somebody previously blocked).

noun

1. The removal of a block or blockage.

   “Look for every opportunity to create an entry to the weak hand. Spectacular unblocks are often needed.”

   “Control messages, such as requests/forwards/unblocks, are often shorter as they need to carry the address and some header information, while data messages, such as responses, are cache-line sized with some extra header information.”