HD TV Box Problems

I recently moved into a new apartment and set up my cable modem and HD cable box using a splitter.  They both worked perfectly the first day but now two days later the HD TV box does not work at all.  It doesn't allow me to watch any channels or even go into the settings function.   The modem still works perfectly on the same outlet.  Is the cable box faulty or is the connection to my apartment not allowing for the signal to be split?