This might be a silly questions but do you have wood cutting turned on for some dwarves? All your woodcutters might have been killed in that undead siege.
Also check they are not restricted to any burrow.
Yes, some have wood cutter turned on. There are no burrow either.
...The tree cutting was going fine until this afternoon, when I got attacked by undeads and lost 75% of my peoples.
If you assigned these guys to a military squad and gave them a Move or Kill order, that is the problem. It is a conflict between military and civilian labor uniforms. To fix it, remove them from the squad, unpause a moment, then disable and re-enable their woodcutting skill again. You can put woodcutter. miners, and hunters in a squad but as soon as they are given a squad order or given a barracks to 't'rain at, they stop being willing to mine, woodcut, or hunt.
I just tried it, it didn't work.
Do your woodcutters have axes in their inventory?
If:
a) your previous woodcutters were killed or deactivated (freeing their axes)
b) you drafted some dwarves (who claimed the free axes as weapons)
c) you appointed some new woodcutters (who can't find a free axe)
then your axe-less woodcutters won't be able to actually perform any woodcutting jobs. In that case you'd need to produce more axes or wrest the existing ones away from your military. Note that wooden training axes no longer work for tree cutting.
I tried the mentioned above, disbanded all squad, removed all woodcutting jobs, then put thm back on. I also checked if there were axes, and I can find 2.
But I just bought three axes from the merchants in my village, it seems that it was the problem. I can't believe it, for some reason there are two axes in the village, yet nobody could find get their hands on them. Well, sorry I bothered everyone with that, and thank you. That also mean that the migrant wave I had earlier didn't bring a single axe on 16 migrants.
While i'm at it, if anyone knows a way to stop the trees to "cave-in" every time I cut them, I'd be glad. I just lost 3 more people in that tree-cutting session.