I see too many threads about "my light won't come on" where people just do not know the basics of what to do in that type of situation, so here it is in a nutshell:
BASIC FLASHLIGHT TROUBLESHOOTING PROCEDURE:
1 – Check the battery polarity (most often positive towards the head, but not always)
2 - Make sure all parts are screwed in all the way and remove / reinstall your dropin if applicable while inspecting for broken parts.
3 - Try a new non-rechargeable battery (and new bulb if it’s an incandescent)
4 - Clean all contacts: inside the head, spring(s), tube ends and threads. (Rubbing alcohol and a Q-tip work fine or this)
4A - For twisties and lights that have erratic behavior: check if your o-rings are not creating too much friction (too dry or too big) or not enough tension (too small). Big o-rings can prevent contact, small o-rings can make contacts happen when not desired, causing flickers or undesired mode changes.
5 - Remove the tailcap and short the end of the battery with the end of the tube (a paper clip works just fine).
5A - If this works: tighten the tailcap retainer ring (note that some are reverse threaded).
5B - If tightening the retainer ring doesn't work dismantle the switch, clean all contacts inside the switch area
5C - If this does not fix the problem, but the light comes on when you short the battery and the tube end, you most likely have a defective switch.
6 - Contact you dealer. If the dealer warranty is over, contact the manufacturer. Some dealers and manufacturers will honor the warranty for longer than stated or will provide replacement parts for a reasonable fee and sometimes even for free.
Don’t just give up on a light or brand just because your light didn’t come on.
As a standard maintenance procedure, you should lubricate your o-rings (see the comprehensive grease and lube thread for more details) and clean your contacts on a regular basis (at the very least once or twice a year up to once a month, depending on your usage.) Once a year, I check that my retainer rings are screwed in properly and I basically never have any problems.
Let me know if I forgot anything and I invite you to share your tips & trick or stories of a light that neede a simple maintenance to get going again