SIA has cut capacity to Americas, so MAN-IAH is one of the routes affected. They happened to cancel the flight you were originally booked on, and offered to rebook you to the next avaialble flight which is 2 days later. You can call them up and ask for a refund. or re-route to any toher AME points without additional charges.
https://www.singaporeair.com/en_UK/u.../ne0822-221122
Flights SQ52 and SQ51 that serve the Singapore-Manchester-Houston route will operate thrice weekly instead of four-times weekly from 28 March 2023. SIA will launch a new twice-weekly service between Singapore and Manchester, SQ302 and SQ301, operated by the A350-900 long-haul aircraft, from 2 April 2023.