Hey man, I'm not exactly experienced and am pretty rookie like you (judging by your number of posts and topic), so my advice may not be top notch but hopefully it will benefit us both.
Like you, I have been friendzoned and i am the sort of "Fun, gay best friend" type of friend to some girls who I am friends with so I think I can relate. It's best to try and figure out where you went wrong so you can correct it.
[u]Where I think you went wrong.[/u]
I'm sure that the reason you are friendzoned is because you have made this girl comfortable and close to you without making her attracted to you [i](I know she said she "lost feelings all of sudden", implying she was attracted to you at some point. But unless you done something so unattractive it repulsed her, I doubt she really was attracted to you and probably lead you on or didn't want to shoot you down too harshly)[/i].
Most common ways of getting friendzoned by not building attraction:
+ Constantly communicating without making a move which leads to her getting bored.
+ Not taking charge / Her calling the shots (Her deciding when the conversation stops, her inviting you to events, ect).
+ Being like her little dog who waits on her hand and foot. (Acting like your world circles around hers)
These are just some things that you may have done, It could have been anything.
[u]What to do next?[/u]
I'm guessing the reason she's ignoring you is due to it being awkward or she got bored with you, either way... Fuck that bitch, I know getting over someone is hard (as I said, I've been in a similar situation with someone who I have 2 classes a week with) but if she's ignoring you then trying to keep talking to her will just bug her to the point you feel like shit and look needy in front of everyone. Time will help you get over her and trying to find her flaws and focus your attention on something else like trying to improve in PUA would probably help and benefit

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I'd advice you to get over her but if you are going to try and win her over here's some stuff that [i]might[/i] help. I wasn't very successful in my most recent experience so only try what you think will work. If she's ignoring you try and talk with some mutual friends of yours (If yous have any), If yous happen to hang out together from time to time in a group then there will be moments when there's just the 2 of you and making natural, NON-AWKWARD conversation/banter is just the right thing to do [i](even if there isn't there will be moments where you can look attractive by being an important member of the social circle and being fun).[/i] You should also remove that comfort and closeness, stop texting her and refrain from trying to talk to her (only talk to her specifically when she comes to you or yous are alone together), doing this will also help by not chasing her because chasing her will make you look like a pussy where as you need an attractive quality, such as looking like a boss.
If you don't have any mutual friends where you can get to hang out around her then maybe when you see her around school (or work, don't know how old you are, I guessed school haha) just act like you happen to have been in the same place, if she's choosing a drink at the shop or vending machine stand behind her and cover her eyes, or nudge her out the way with a gentle hip bump and playfully say "my turn" and try using the machine, she'll laugh and try pushing you out the way. If you do stuff like that then you are being fun and you aren't chasing her, your just getting a drink and she happened to be there too. Its just an example, try and do little playful things like that when you see her around, but don't hang around afterwards waiting to talk to her, be busy and just a quick playful move as you walk past or doing whatever your doing (you'll look fun and not like your world evolves around her. She may even text you after you leave with something stupid like "Heyyyyy! bumping me was rude

, I'm gonna beat you up next time!" or somthing stupid like that which allows you to be playful and talk back without looking needy) [i](by the way, one playful thing to avoid is tapping her on one shoulder then walking away past her other shoulder so she looks round trying to see who it was, when she sees you and realizes you will just look silly and desperate for her attention. plus it isn't even that funny. But its your choice, that's my opinion on the shoulder tap haha. [/i]
Hope this helped man, writing it has certainly been an eye opener for me

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As I said, I am still a rookie too so don't take my word for it and think this is fail proof but in theory I think this could help and it certainly didn't do any harm in posting it.