kids as they grow will have several 'falling back' periods. they take 2 steps forward and 1 step back.
first of all, is there a nightmare happening? night terror? sleepwalking? point being, is your daughter really awake, or is this a sleep disruption where she isnt fully awake? if shes up for 2 hours im would assume she is awake..
has anything tramatic happened in your family lately? even if a child cant understand whats happening, they DO understand when you are stressed or worried, and that could be interrupting her sleep, possibly with nightmares.
if there is something in her room that scares her, hide it from view. if there is an open window, shut the shades. if she needs a nightlight put one in there so she can see. i know that even I am afraid if i cannot see - whether it be deep water or darkness.
most of all, remember that the more you can be there for your daughter as she goes through this, the more confidently she will get through it. dont treat it like an annoyance, or a bother, even if it is, cuz then your daughter will get the impression that her feelings arent important.. its a stretch for a loving mother to give that impression, but kids dont understand that mom needs sleep, they understand that there is something they need or want... does that make sense?
now, within reason. if she wants to be up and playing all night long, that obviously is not acceptable. even the 2 hours i would figure is not acceptable. if needed, place her matress on the floor in your bedroom. she may be more comfortable closer to you for a while. if shes been trying anything new or learning anything new, maybe take a step back with her and try to avoid that thing or deal with it as little as possible. new things are sometimes hard for kids, as they gain independence, also can come some anxiety that mom is gonna stop helping or stop being there, which isnt true, but it can sometimes feel that way to a kid.
anyway, just have patience, and loving responses to your daughter and hopefully she will get back into her normal routine. make sure that she is safe if she is staying somewhere else on a regular basis, and make sure that she always has your loving arms to catch her when she needs that time.
maybe shes reacting to the new baby. it sometimes takes some time to get used to baby being around, and i spose by 6 months, it can start to get annoying, this person taking all moms time away. make sure that you and your daughter are getting time during the day where its just you two.