If I buy a home that had a land contract prior to my buying the home and the people have not paid in 2 years, and no one has been in the home for 3 weeks now can I start living in it today?

A land contract purchase means that the owner of the property retained ownership until the contract was completed. If the contract buyers did not complete the contract, then they were in default of the contract and the owner should have exercised the contract terms in case of default.

If you are purchasing this home outright, then you can move in as soon as the closing has been completed. If you purchased this home while the contract buyers were still in residence, then you accepted them as tenants and you have to evict them or take possession according to local laws. If they have moved out and handed over the keys, then they ceded possession and you can move in immediately. If they just abandoned the property, you may have to make sure legally that you can take possession.

You really did not give us enough info to give a good answer (any question on law needs to include your general locale as laws vary by area).