、！。
What are we gonna do about this shop?! It's completely dead in here.
Literal
How-do-[nominalizer-の], this [polite-お] shop! Completely cuckoo-[subj-が] is-crying-[regretfully-ちゃってる] [isn't-it-じゃない].
どーすんの is the spoken slurring of どうするの 'what are (we going to) do.' 完ッ全に uses a small ッ between 完 and 全 to mark emphatic gemination — a written convention for 'COMPLETELY' in an emotional voice. 閑古鳥が鳴く (literally 'the hermit cuckoo is crying') is a set idiom meaning '(a shop / venue) is completely deserted' — the image is of a place so quiet only a lonely cuckoo's call can be heard. じゃない with falling intonation is the confrontational 'isn't it!' rather than the rising-intonation tag question.