Business & farm

Ok.  Responses are in line with what I thought, just don't want to assume too much here.

1. What has happened is that your business went from a multi-member LLC to a single member LLC (SMLLC) when you bought out your partner.
2. You have a final partnership return that WAS due January 15th.  Attempting to file a late extension will do you no good.  Just get the form 1065 filed as quickly as possible.  You will most likely receive a notice from the IRS regarding late filing along with some penalty.  Respond that you now realize the return was due sooner as a result of the partnership termination, however, all member's have received their K-1's and there was no intent to avoid or evade the tax laws.  It was a simple misunderstanding.  Any correspondence with the IRS should be sent certified mail return receipt requested.
3. Your situation is covered by Revenue Ruling 99-6, of which I have attached a link.  Your facts are situation 1 in the revenue ruling.  Read through this a couple of times to make sure you understand the mechanics of what is going on post LLC termination.  You will note that depending on the assets held by the LLC, you could have a bifurcated basis in each asset post termination.
4. https://www.irs.gov/pub/irs-drop/rr-99-6.pdf
5. As a member in an LLC taxed as a partnership, you need to maintain a basis schedule.  This is your investment in the LLC.  Your basis begins with your capital contribution and is updated annually for the applicable lines on your K-1.  Your basis is very important as it determines whether losses are currently allowable and also your gain or loss upon sale.  Attached is a link to the instructions for the 1065 K-1.  Page 3 has a discussion of how your basis is adjusted annually.
6. https://www.irs.gov/pub/irs-pdf/i1065sk1.pdf
7. You now need to determine the allocation of the purchase price you paid for the 50% of the business.  Since an LLC taxed as a partnership is a pass-through entity, you need to allocate that purchase price to the various assets.  
8. Then these assets are distributed out to you.  Once again you now have to allocate a basis to the assets, however, the total allocated to the assets can't exceed your basis in the LLC.  Yes this is somewhat confusing, but it is the way it needs to be done.  There are specific rules in the regulations that deal with this based on various facts.  You may want to consult with a tax professional to get this right.
9. Finally, your partner needs to determine their overall gain or loss on the sale of their interest.  Additionally, if you have what is called "hot assets" (Section 751 property) some of the gain may need to be reclassified as ordinary.  The hot assets would be depreciable property (depreciation recapture), receivables if you were cash basis and inventory.  Once again, this can be tricky and the help of a tax consult would be beneficial.  The overall gain or loss does not change, it is just the classification (characterization) most likely will be changed.  Example:  Overall gain of $10,000.  Section 751 property gain $5,000.  Here the partner will have $5,000 of ordinary income and $5,000 of capital gain.
10. Your LLC just continues on; no longer a multi-member LLC, but just a SMLLC reported on your Sch C
11. May be overwhelming, but this is the world of partnership tax.