It would help if we knew what platform you are playing Machinarium on. ( there are different quirks to each one of them )
But, no matter what platform you play on, there is a walkthrough available in the upper right hand corner of your screen ( click on the book and kill the dangling spiders to unlock the guide). Look and you'll see that you get the plunger
AFTER you crawl out of the sewer in to the guard room.
Oh, and if the rodent has stolen your "broom-key" before you opened the next sewer cap then, yes, you'll have to restart that level. (hopefully you saved BEFORE you went down the sewer!)