Queen did not technically "end," but the original lineup effectively ceased active performances and recording after Freddie Mercury's death and John Deacon's subsequent retirement.
The following factors contributed to the shift:
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Freddie Mercury's Death: On November 24, 1991, Freddie Mercury died due to bronchopneumonia, a complication of AIDS. Mercury's distinctive vocals and flamboyant stage presence were integral to Queen's identity, making it difficult to continue in the same format without him. His last album with the band was 1995’s Made In Heaven which featured vocals recorded prior to his death.
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John Deacon's Retirement: Following Mercury's death and the release of Made in Heaven, bassist John Deacon became increasingly disillusioned with the band and the music industry. He retired from performing and recording in 1997, essentially removing another key member of the original lineup.
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Continued Activity by May and Taylor: While the original lineup was no more, Brian May and Roger Taylor continued to perform and record under the Queen name, often collaborating with other vocalists like Paul Rodgers (as Queen + Paul Rodgers) and Adam Lambert (as Queen + Adam Lambert). However, these projects are distinct from the original Queen lineup. They should be thought of as a continuation by two remaining members of the band versus an "end" to Queen altogether.
Therefore, the closest one can get to an "end" of Queen is the loss of Freddie Mercury and the retirement of John Deacon, which marked the effective end of the original lineup's active period.
