Norodom Sihanouk was crowned King of Cambodia on 3 May 1941. At the time, Cambodia was a colony of France, so he was not really a head of state yet. But he remained king, and therefore head of state, of Cambodia when the country became independent on 9 November 1953. He then abdicated on 2 March 1955.
After that point, Norodom Sihanouk was appointed prime minister and effectively ruled the country, but he was not formally the monarch or the head of state: the monarch and head of state was his father Norodom Suramarit.
After his father died on 3 April 1960, Norodom Sihanouk's mother Sisowath Kossamak became Queen of Cambodia. Her son Sihanouk became head of state, but was not formally crowned king.
On 9 October 1970, following a coup, Sisowath Kossamak was deposed as queen and Norodom Sihanouk was deposed as head of state, and Cambodia became the Khmer Republic. Norodom Sihanouk went in exile.
In 1975, after a civil war, Cambodia became Democratic Kampuchea. Norodom Sihanouk was formally the head of state of Democratic Kampuchea from 17 April 1975 until April 1976, after which he was under house arrest, then went again into exile.
Following the fall of the Khmer Rouge regime, Norodom Sihanouk returned to Cambodia in November 1991. The constituent assembly declared him head of state again in 1993, and on 21 September 1993 a new constitution went into force, making Cambodia a kingdom with Norodom Sihanouk as its king. He then reigned (with very limited political power) until his second abdication in 2004.
If you only count the times when Norodom Sihanouk was formally a monarch, there was a 38-year gap (1955–1993). If you count all the times when he was head of state, the largest gap is a less impressive, but still significant 17 years (1975-1993).
(Apologies for only referencing Wikipedia, but I don't think any of the facts that are relevant to this question are in dispute. If I have inadvertently misrepresented Cambodia's complex history, please correct me.)