If the buildslave cannot connect to the buildmaster, the reason should be described in the twistd.log logfile. Some common problems are an incorrect master hostname or port number, or a mistyped bot name or password. If the buildslave loses the connection to the master, it is supposed to attempt to reconnect with an exponentially-increasing backoff. Each attempt (and the time of the next attempt) will be logged. If you get impatient, just manually stop and re-start the buildslave.
When the buildmaster is restarted, all slaves will be disconnected, and will
attempt to reconnect as usual. The reconnect time will depend upon how long the
buildmaster is offline (i.e. how far up the exponential backoff curve the
slaves have travelled). Again, buildslave restart BASEDIR will
speed up the process.