The 3100's have been known to have the centre lead tear off the circuit board inside the units, which is a quick and easy repair.
To test for this without opening the up the unit, while connected to your TV and satellite dish, and in the signal strength meter window of the menu, place a small amount of pressure upwards with your fingers on the coax going into the receiver from the satellite dish. The upwards pressure pushes the centre lead down inside the unit, onto the circuit board, if you get signal when doing this, (and it doesn't take much pressure), it can be fixed with a re-solder job.
Otherwise see the Retentions thread at http://www.digitalhome.ca/forum/show...=48593&page=43
as you may even be offered a free rental.