。
I must have dropped it.
Literal
Drop-[regret-てしまった] [must-be-にちがいない].
Two auxiliaries stacked: ~てしまった (completion, here with regret — 'ended up dropping it') + にちがいない ('must be / no doubt'). 落とす is transitive 'to drop [something]' (intransitive partner: 落ちる 'to fall'). The subject and object are both omitted — only context tells you what was dropped, and by whom.