From the link @Revels shared,
If you sent your cheque over 7 working days ago and haven’t heard anything, please chat to us.
- The date you sent it
- Whether you sent it using freepost or tracked (if you have a tracking number, please share it)
- The amount and currency - we can only accept cheques in UK pounds (GBP).
- The name(s) on the cheque
We’ll look through any cheques we have on hold. If we find yours, we’ll process it and it’ll be in your account within 4 working days.
If we can’t find it, we might ask you to get in touch with the sender to get a new cheque.
Go through that process, and wait for the money to arrive.
If you aren’t happy with the way Monzo are behaving, make a complaint. As @Ordog says, you may get a few quid extra for your trouble if Monzo have done something wrong.