ive found in working with the lower end "in home" machines, that they tend to jam up sometimes.
my first suggestion is to check how your bobbin is wound and loaded into the machine. if fiddling with that doesnt help...
check out your tension, if the upper thread is the problem, its mostly likely the tension, if the lower thread is the problem, its still a likely culprit.
from there, if neither of those is the fix, if you can take out the bobbin casing and/or throat plate and clean the machine out with some compressed air (comes in a can, like what is used to clean out crumbs from a computers keyboard)
ETA...OH! and as the above poster mentioned, check out if youve accidentally gotten it threaded wrong...that could be causing some major issues as well, and its a simple fix.