Question

Where art thou, Muse, that thou forget'st so long

Answer 1

Answer:

To express that the author's 'Muse' (or inspiration) is gone

Explanation:

"Where art thou, Muse, that thou forget'st so long

To speak of that which gives thee all thy might?

Spend'st thou thy fury on some worthless song,

Darkening thy power to lend base subjects light?"

I can sorta guess that this is asking something like "Where are you Muse, did you forget about me for so long?"