This afternoon my Monzo card was rejected when I was buying petrol at our village garage - something I’ve done dozens of times before. As a check I tried to draw cash from the ATM at the same location - same result. Were the Monzo systems down?
No reported downtime today. Perhaps have a chat in app to see if they can pinpoint the cause? 
No known outages between us and Mastercard today. 
Was it a contactless transaction? And have you used the ATM before (not all ATMs work with MasterCard)?
I’ve used my Monzo card many times at the till and at the ATM. But the staff were muttering about similar problems with Lloyd’s cards earlier, so maybe it was a transient issue with the shop system.
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Maybe a communication problem? If it keeps happening I’d order a new card.