On this model there is no fuse in case you test to high of a voltage. So My thoughts are maybe the small spring fell out when changing the batteries or ( I am not being smart ) you put the batteries in backwards. Also try another set of batteries and of course a circuit you know is live. Mine is 5yrs+ old and have never had a problem with it. If tis doesn't work they aren't that pricey I would just get another one.