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

Encyclopedia: All of a sudden

• unregularity
• unregulated
• unrehearsed
• unrejoicing
• unrelated
• unrelatedness
• unrelaxed
• unreleased
• unrelenting
• unrelentingly
• unreliability
• unreliable
• unreliableness
• unreliably
• unrelieved
• unremarkable
• unremarkably
• unremarked
• unremedied
• unremitting
• unremittingly
• unremorseful
• unremunerative
• unrenewable
• unrenewed