Just after I bought my first Europa bass in 2005 I decided to get some fretfx LED's and they have been great and only yesterday I changed the batteries for the first time for some new ones. But as it happens I wasn't concentrating when I did it and put them in the wrong way round. the lights came on briefly then stopped and when I put the batteries in the right way round i get no light from them.  
 
Have i really trashed them?  
Jazzyvee

I installed FretFX LEDs on a bass and they recommend storing them with the battery reversed in the clip.  Probably mine are a newer design (I got them in January), but I wouldn't think a button cell could possible destroy them.
 
John

Mine is the older one with two AAA batteries.
JAzzyvee

unless there is something special w/ a circuit before the actual LED's, it should not hurt anything.  A LED is a diode, a simple electronic component that allows current to flow in one direction.  Check the batteries.  They might be bad.
 
Good Luck
 
Stephen

Unless there was some sort of physical damage to the contacts for the batteries while you changed them, I can think of no reason why having them in backwards would cause any damage.  I've had the 'tab' or the 'spring' contact point break on a couple things in the past.