I am running a game and I made a thread for the players to vote on something. I made the thread a notice so the it would stick to the top, but it seems it unexpectedly closed the thread as well.

I tried making it no longer a notice but it still is closed. The faq tells you how close a thread* but not how to open it.

*Note, the faq does not tell you that making a thread a notice will close it.

So can anyone tell me how to open a thread please?

Making something a notice automatically closes it, but it's no big deal.  If you know how to un-make it a Notice, then you're halfway to opening it to posts.  Just go back there, recheck the Notice box if you want it to stay a Notice, and then uncheck the "Close Thread (forced for archives)" box.  Viola!  An open Notice thread.

I don't see such a box, just group select, bump, notice, and cancel (and the formatting options of course)

Odd.  What you should see under the Edit Thread link  is:

New Group:
Notice (fixed to the top)
Archive Thread (shows all private lines)
Close Thread (forced for archives)
Bump Thread

No formatting options or cancel.

What are you clicking on to see group select, bump, notice, cancel and formatting options?

Wait, I think you're editing the first post in the thread.  What you need to do is open the thread, then click on Edit Thread link in the header bar instead.
This message was last edited by the user at 11:15, Mon 09 Jan 2017.

Oh, neat. I didn't know about that. Might be a good thing to clarify in the faq.

There's a large section in the Help about the thread options available to GMs: /help/?t=help&page=threadsposts