«فَوَجَدْتُ يَدَ الضَّيَاعِ قَدْ غَالَتْ إِحْدَى الرِّقَاعِ»
I found that the hand of loss had taken one of the slips.
When she recounts the slips, one has disappeared. The “hand of loss” personifies loss as an agent capable of taking something. This is an instance of personification/metaphorical expression. The literal event is simply that one written slip cannot be found.