Low volume ,Muddy PA .Is it the desk?

Morning folks ,last night the PA sounded dross ,Middy ,Bassy no volume or tops to speak of.I had to face the monitors to the audience they worked OK luckily so we got away with it.It seems like something has gone wrong with the 802 mixer.Any ideas please ?
System: 802 mixer
1 x TS18 Alto sub ,
2 x Alto TS115 15s all powered.
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