Unplug the power cable for 30 secs?
If you're going down that route, make it at least a couple of minutes.
Do you have a "friendly" neighbour with SKY - it's easy enough to pop round with your receiver and try it on a known, good signal. If this proves your box to be working, then you'll have to look more closely at your dish/LNB. Pay particular attention to any joints between brackets and hardware - new shiny metal can be a handy indication of movement. You
may be able to sight along the LNB arm (from underneath) and pick some point in the distance, then do the same thing at your neighbours - this would show any substantial movement. The elevation is just as important - a spirit level with an adjustable "bubble" held against the dish face could be used to compare with another installation.
As for LNB's - yes they do fail, but not always completely. There are 4 states involved - high or low band, and vertical or horizontal polarity. Losing any one of these will only affect certain channels, and not usually in any particular order. Please note: Disconnect the receiver from the mains before fiddling with the co-ax connectors - shorting the output can damage it. For the same reason make absolutely sure that no stray strands of the braid are touching the inner conductor if you check the plugs. "F" connectors don't have a separate pin - the cable inner does this job, and if it either 1) isn't long enough to make contact inside the socket, or 2) gets bent back during insertion you won't get a signal. Unlikely with a previously working installation, but worth checking anyway.
Failing any of this, you'll have to get an installer to come round and check things for you.