Land Contract Wrap gone Bad

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Had a deal I intended to wholesale about 18 months ago but could not pin down a Cash buyer for the property.   I wound up buying the property on a land contract and then reselling it immediately on a land contract with an increase in monthly payments to collect a monthly spread, everything was fine for about 15 months.  The vendee buyer / end buyer has not made a payment in over three months.    This is a new situation / problem that I'm not accustomed to so any pointers would be helpful.