Without knowing what type of detector you have this is a little difficult.
I'm going to assume you have a Dual Detector of some sort, usually a PIR & Microwave in one unit.
If you have been Spring cleaning or similar perhaps you have inadvertently 'masked' the microwave side.
That means that the microwave took your close up cleaning as an attempt to 'mask', cover/stop the detector working correctly, as a result it 'switches off' the microwave side of the detector leaving it working only on the PIR side.
If what I have said rings true, the only way to reset the detector is to remove the power to that detector for 30secs or so to allow it to reset back to its normal condition.
How you need to go about that is another story.
Please post HERE again to let us know the result or to ask for additional help.
Good luck.
acorn
March 2011