Most likely the LNB on your dish is getting hot and drifting off frequency. Try cooling it with the garden hose and see if the signal returns. If it does then it's time for a new LNB. You may be able to get the signal back temporarily by adjusting the LNB freq up or down a few Mhz in the setup menu. Loss of signal over time is usually due to a LNB failing as it ages.
Biggles is right, The cause could be your LNB which overtime does age and become unreliable, you can get LNB's pretty cheap on Trademe or your local installer from a store. We have helpful guides if you wish to do this by yourself here: http://freeviewforum.co.nz/forum/topics/freeview-satellite-adjustin...
Adjusting the frequency is a short term fix and overtime will fail so I recommend you seeking a new LNB.
I hope this helps! If you want any further information let us know! :-)
thanks for the reply guys, iv tried the frequency fix but in fill sun it disappears again,what is the LNB?