stargazing-my hen unwell-help!

My bantam buff orpington Elspeth has "stargazing" syndrome. her head tilts back lots as if to look at the clouds/stars....she is not standing up now either. I am really upset....The only hope is that her appetite is still huge, been hand feeding her ex-bat grit and tonic. Can anyone give me advice? I read thiamine is good, but how do I get that? Thankyou x