I tried to ask a Theoretical Physics question, then I found that the Theoretical Physics Stack Exchange site has been closed.
Now what should I do?
Yep ask here, we like theoretical physics too ;-)
Some questions that have been on the former theoretical physics SE site or new ones, that would have been appropriate for it, are labeled with the "research-level" tag.
Here you can see (beside the Astronomy questions which we have saved too) a list of all the TP.SE questions that were imported. We have successfully saved all of them I think.
What went wrong with TP.SE was discussed for example at our meta by questions marked with the tag "site-salvage". In my opinion, the basic proplem was that the SE network does not allow for smaller, high quality content sites, which are of interest to a smaller community and therefore have lower, traffic, a slower growth rate, etc ...
As described here, the content of the former Theoretical Physics SE site has been made human readable, editable, and reclaimable by its owners such that these discussions can continue or be taken up again in principle.
Please read the Stack Exchange blog post "When a Site Grows Quiet" about the fate of TheoreticalPhysics.SE and Astronomy.SE. If, you still have questions, then meta, not the main site is the place to bring them up.
Let me repeat in case it is not clear from the other answers that:
$^1$ Except for a handful or two, which were migrated to TCS.SE.
Theoretical physics was a site which attempted to produce a class-structure of "high end" and "low end" questions, so that people who are professionals won't have to mix with the hoi-polloi of students and laypeople. It is both taxing in terms of time and humiliating in terms of standing for professionals to be on an equal level with all others. They don't like it, as they spend a large fraction of their lives trying to acquire some sort of social standing that makes their proclamations more attention-worthy than that of the average person.
I don't like this, it really doesn't sit well with me. I think the social mechanism is no longer necessary, now that people can easily answer easy questions and easily discuss the tough ones. The best questions of a theoretical sort are often those which are so outlandish, that only outsiders are allowed to ask them at first. "Is light a particle?" would have been such a question in 1905, and Einstein's early realization that this is an important line of investigation made him a bit of a pariah for 15 years, as nobody else was able to see through the fog of political rejection of this idea, left over from the 18th/19th century form of the debate, which was closed by Fresnel.
The openness has its problems, there are always more annoying questions which are not particularly informed, but it has the advantage of no political hierarchy to censor things, and the gains are greater than the loss in my opinion. This is why it is not a disaster that TP was closed, and it is also part of the reason I always felt uncomfortable there: there was a lot of throwing around of fancy terms on that site, although much of the topics overlapped. Also, it was hard to write on that site, as the requirement of separation from this site required you to have something abstruse in an answer, so that you don't muck up the discussion.
This is academic conventions for writing, and it is not really compatible with free exchange of ideas. Generally, it doesn't harm physics too much, since people eventually get the hang of the calculations and jargon, and then blend into a field. But this merging with the herd of professionals comes at the cost of intellectual independence, and there is this constant tension.
So TP was closed, and merged with this site. Nearly all the professional physicists left. This is sad, but one can soldier on. The only thing is to arrange the upvotes appropriately, so that difficult research questions have a large number of upvotes, while easy homework questions are answered with a one-liner or closed.