1. Without warning
   - all at once 
2. Happening unexpectedly
   - suddenly, of a sudden

Encyclopedia: All of a sudden

• clear-eyed
• clear-headedness
• clear-sighted
• clear-thinking
• clearance
• clearance sale
• clearcole
• clearcutness
• cleared
• clearheaded
• clearing
• clearing bank
• clearing house
• clearly
• clearness
• clearstory
• clearway
• clearweed
• cleat
• cleats
• cleavable
• cleavage
• cleavage cavity
• cleave
• cleaver