His unexpected resignation __________ in the office.

Catch somebody unawares/ catch somebody off-guard/ catch somebody on the hop (idiom): 
English Meaning - To do something when someone is not ready for it and is not able to deal with it. 
Bangla Meaning - কোনো কিছু করা বা ঘটে যাওয়া যা অপ্রত্যাশিত এবং সামাল দেওয়া কঠিন।

Complete Sentence - His unexpected resignation caught everyone off-guard in the office.
Bangla Meaning - তার অপ্রত্যাশিত পদত্যাগ অফিসের সবার জন্য অপ্রত্যাশিত ছিল।  

Other options, 
• Throw in the towel - Stop trying to do something because you have realized that you cannot succeed. 
• Burn the midnight oil - To work late into the night. 
• Turn over a new leaf - To start behaving in a better way.

Source: 
1. Cambridge Dictionary. 
2. Accessible Dictionary by Bangla Academy.