Have a read of this as background reading:
Brendan Burgess' guide to Mortgage Repayment Calculations
Case 1
To make it simple, assume 1% interest a month on an interest roll up loan i.e. they don't require any repayments
Balance 1 January: €100,000
Balance 1 February : €101,000 ( €100k +1%)
Balance 1 March : €102,010 (101,000 +1%)
Balance 31 March: €103,030 (101,000 +1%)
You were not due to make any repayment,so there are no arrears.
Case 2
Now assume you make €2,000 a month repayments
Balance 1 January: €100,000
Balance 1 February : €99,000 ( €100k +1% -€2,000)
Balance 1 March : €97,990 (99,000 +1% -€2,000)
Balance 31 March: €96,970 (97,990,000 +1% -€2,000)
Case 3 - You are supposed to pay €2,000 a month, but don't pay anything
Because you paid nothing, your mortgage balance will be the same as Case 1 i.e. €103,030
However, your arrears balance will be €6,000 the sum of the missed payments.
If you now pay off the €6,000, the balance will fall to €97,030