A vacuum that isn't suctioning could be having any number of problems. There could be a clog in a filter, which is one common reason why this happens. \
There could also be a clog in a hose or some other blockage that is preventing the vacuum from suctioning. A tear in a hose can cause the suction to fail as well. \
If the vacuum is not suctioning but spitting out, check to see if the bag needs to be changed or the debris tank needs to be emptied. \