She's gone from friendly to just plain cold and ignoring me, what to do?

OK so there this girl at work we got on fairly well but now she just completely ignores me, She is on the shy side but it makes no sense every1 else is fine towards me and I've noticed she talks to everyone else throughout the day but she will just act as though I don't exist at all :( only talk to me if she has to about a work related problem but she will cut it short, before she used to be really into my jokes and smile and stuff, I started to feel best way just to ignore her to
She's gone from friendly to just plain cold and ignoring me, what to do?
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