LNB's can fail at any time and come in various types. The mount that attaches the LNB to the arm is usually rectangular. LNB's and mounts are usually supplied separately. If you look at the Maplins site it will give you an idea of the prices click here but you may be able to get them cheeper via the web.
Note: check the cable for any faults. Cracked/split cable will let water in causing problems. The F connector at the box or dish can become loose also causing this problem and last the box can also fail.