What to do: Look into the ceiling.
If the joists are perpendicular to the old wall and their ends sat on it (probably overlapping it slightly), it was a bearing wall—i.e., it held the weight of the house on its shoulders. Build two temporary walls, one on either side of the removed wall, by securing 2x4s to the floor and ceiling and inserting a stud between them under each joist. Cut the studs 1/8 to 3/16 inch longer than needed, says This Old House general contractor Tom Silva, and wedge them in place with a sledgehammer. Then call a contractor or engineer, who should be able to cut back the joists and install a beam in the ceiling, at
which point you can take down
the temporary supports and enjoy your open floor plan.
How to keep this from happening to you: Do your detective work before you demo the wall. For the most part, an interior wall is load-bearing if it is perpendicular to the joists or rafters above it, rides directly over a basement beam, and has an exact match on the story above and/or below. If you’re unsure, hire a contractor or engineer to look at the house, or take the blueprints to your local building department for assistance (and be prepared to apply for a permit for the removal job).