Besides a few common phrases to express disappointment, here comes a great video with a lot of examples used in different contexts to teach you how to use "Damp Squib"
The magic show last night turned out to be a damp squib.
It was a damp squib.
But then the rain came and turned the party into a damp squib.
The whole thing turned out to be a damp squib.
It turned out to be a damp squib this year.
Is there a similar expression or idiom in your language?