I wouldn't be too worried yet

As has already been said, onionland is still working. I haven't had any problems with the official tor checker site until today, I imagine they're just having normal Internet problems every website goes through from time to time (it's not working in my non-tor browser either).

I'm not sure about the other sites, but, if some source tells you your IP address is different than the one your ISP gave you (that is, you get a different IP address when you use one of those online checkers from a browser that is tored and a browser that isn't), it should be safe to assume you're using tor (unless you changed your proxy settings and are using a non-anonymizing proxy without realizing it, but that is user error).

I'm not sure exactly how it works, but the tor network publishes a list of exit nodesthis is how the online checkers know if you're using tor (you're using tor if the IP address it sees is on the list of tor exit nodes). This isn't failsafe, I've had it fail on me before when I definitely was using tor, I imagine I was on a rather new exit node that had not made it onto this list yet. It is possible that some system responsible for compiling this list (if it is not done right on the tor network) has gone down (perhaps running from the same place as check.torproject.org), and these other sites are unable to get information from it.