Faulty connection of some kind - check the headlight that is not working and work backwards checking each connection, especially the earths.
 
I'd be making sure that the fuse or the bulb hasn't blown.

:5biamwithstupid:
Low beam turns off when high beam goes on. Might be as simple as stuffed bulb. Hope you didn't touch the glass of the bulb if it a replaceable type
 
Mine does that now and again. I pull the high beam fuses out,put them back and it all works for a while. Really must get round to fixing it properly.
 

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