urplötzlich
all of a sudden
Completely unexpectedly and very suddenly; without any warning.
Mitten im Sonnenschein fing es urplötzlich an zu hageln.
In the middle of the sunshine, it suddenly started to hail.

suddenly
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all of a sudden
Completely unexpectedly and very suddenly; without any warning.
Mitten im Sonnenschein fing es urplötzlich an zu hageln.
In the middle of the sunshine, it suddenly started to hail.

The prefix 'ur-' serves here as an intensifier for the adverb 'plötzlich' (suddenly). It emphasizes that something happens extremely suddenly and completely unexpectedly. You also find this prefix in other words such as 'uralt' (very old).
Das Auto kam urplötzlich von rechts.
The car came from the right all of a sudden.
As an adverb, 'urplötzlich' (all of a sudden) can stand in different positions in the sentence. It often appears at the beginning of the sentence to emphasize the surprise, which triggers inversion (verb before subject).
Urplötzlich wurde es ganz still im Raum.
All of a sudden, the room became completely quiet.
'Urplötzlich' (all of a sudden) is stylistically stronger and more dramatic than just 'plötzlich' (suddenly). It is often used in stories and reports to mark a climax or an unexpected turn.
Die Musik hörte urplötzlich auf und alle sahen sich erschrocken an.
The music stopped all of a sudden, and everyone looked at each other in shock.
suddenly
'plötzlich' and 'urplötzlich' both mean that something happens without warning. 'urplötzlich' is stronger than 'plötzlich' and especially emphasizes that truly no one expected it. In normal everyday situations, 'plötzlich' is usually enough. 'urplötzlich' fits better when the moment seems especially surprising or dramatic.

Urplötzlich wurde es dunkel, und alle im Raum waren still.
All of a sudden, it got dark, and everyone in the room was quiet.
Der Chef steht plötzlich im Büro und fragt nach dem Bericht.
The boss suddenly appears in the office and asks about the report.