Updated
If you accidentally unscrew your welder, there is no examine text showing it's unsecured, it can't be turned on, and its fail text says it's out of fuel instead of unsecured. Also the sprite doesn't change, of course, so there's little way to tell.
I had the same issue and confirmed it was utterly fucked by asking another player to try using it - they couldn't turn it on either. I also use fuel cans almost exclusively, but I've only seen it happen very recently.
Things that definitely do not cause it, from my testing
Did you try securing it with a screwdriver?
Amazing. You're right.
So to recap, if you accidentally unscrew your welder when sticking your screwdriver back into your toolbelt...
Great call Blithering
it was unsecured?!
OH WHAT THE FUCK
_AAAAAAAAAAAAAAAA_
Most helpful comment
Did you try securing it with a screwdriver?