What is the adjective for word Suddenly?

There isn't a single perfect adjective for "suddenly". However, depending on the context, you could use:

For a sudden event:

* Abrupt

* Unexpected

* Unforeseen

* Sudden (you can use the adverb as an adjective in some cases)

* Sharp

* Instantaneous

For a sudden change:

* Dramatic

* Rapid

* Swift

* Immediate

For a sudden feeling:

* Unexpected

* Overwhelming

* Startling

* Shocking

The best choice depends on the specific nuance you want to convey.