Hi I have wired 3 montion sensors into my outside light so they will switch on depending on the direction somebody approaches them and they did work fine. However they have now started to stay on all the time and will only switch off if the total power is switched off. What could cause this has it been wired incorrectly I don't understand why it would just change.

If you have had a quick power outages and reset, the lights go into an ON mode. Turn the wall switch off for 2-3 minutes and then turn back on.What happens?

Sounds like at least one is malfunctioning, if the whole shebang worked properly at one time as you say.
Now you have to disconnect all of them and test them one at a time.