Realmente no tienes que razonar mucho esa frase, en el contexto en el que lo pones, tiene un significado literal, viene a decir que estas tan cansado que ya no puedes ni pensar u hacer otra cosa, en este caso es cansado pero se podría aplicar cualquier otro verbo que aplicase como dormir, "estoy tan dormido que no me tengo en pie".

In Literary sense means "I can not think of anything"

It means "You can't come up with something"

Yeah, this means that you are not able to think nothing good to write and in the moment is very difficult to think something to write , maybe the reason is cause you're tired or in other case imagine you don't know nothing of some topic and someone asks you about that, you can say: "No se me ocurre nada porque no se nada acerca tema - I can't say nothing because I don't know nothing of this topic". I hope to be helpful.

In this context means that you are and tired and you can't think anything to write in the text.