What penalties are provided if suddenly the masternode stops working?

If the validator skips 12 blocks in a row, then 1% from the stake of the masternode will be cut (burned), and the masternode will be excluded (deemed inactive) from the validation process until the new transaction is sent for being added to the list of active validators. If an attempt is made to double sign the same block (the data in the blocks will be different), then 5% will be cut (burned) from the masternode’s general stake and the forced unbond (partition) of all delegated funds to the masternode will take place. More information about the responsibility of validators can be found in the documentation https://docs.minter.network/#section/Validators