i too have had this one commissioner once just stop emailing after i'd asked for payment details. :V
it's like... okay we discuss everything; materials, more reference images, how long she'd take, the price, and she agrees to take this commission... then i ask her for her bank details so i can pay her before she started and she just... stopped replying.
our emails had been pretty regular, she'd take four days tops to reply to an email, then all of a sudden, she'd stopped replying for a couple of months.
the next email she sent me was confirming on the color, i responded and she'd replied a few days later saying she'd start soon so i emailed back asking for details so i could pay her..... and then i never heard back from her ever again.
this started sometime in march or april last year, last email i received from her was in september the same year.
i was disappointed, 'cause i heard a lot of good things about her and my experience was nothing near good. :V
i commissioned a costume once before this (and once more after), and i used to know quite a few commissioners in person around that time so i know not all commissioners just drop of the face of the earth after agreeing to take on your project. i however haven't had any never respond at all, though some take a few days to email back, so i'm not sure about that.