The question, "Is there no debt to pay" (line 4), which the speaker directs to his beloved likely alludes to -his earlier manifestations of his love for her
-a dowry promised by her father
-contrition for his absence
-a sum of money he had previously lent her
-gifts he had previously given her

Why are thou silent! Is thy love a plant
Of such weak fibre that the treacherous air
Of absence withers what was once so fair?
Is there no debt to pay, no boon to grant?
(5) Yet have my thoughts for thee been vigilant --
Bound to thy service with unceasing care,
Thy mind’s least generous wish a mendicant
For nought but what thy happiness could spare.
(10) A thousand tender pleasures, thine and mine,
Be left more desolate, more dreary cold
Than a forsaken bird’s-nest filled with snow
‘Mid its own bush of leafless eglantine --
Speak, that my torturing doubts their end may know!