Hi,
Try firmly holding the gauge section of the sphygmomanometer and gently tap it against the palm of the other hand and see if the column re unites.
Here is a link that describes three methods of re joining the mercury column in a thermometer. The principle should be the same for a sphygmomanometer.
http://novalynx.com/store/pc/Rejoining-a-Separated-Thermometer-Column-d40.htm
Here is a link on how to maintain a mercury sphygmomanometer, which may also be of interest to you. http://www.ebme.co.uk/articles/maintenance/344-maintenance-of-a-mercury-sphygmomanometer.
Hopefully this is of some help.