Working on a project that another contractor left halfway through. All the demo was complete when I came to site, now I'm 90% done, and just reconnecting new fixtures/plates ect(I did no electric work while there, just put up some new sheetrock, ceiling texture and paint.) 1 box was left bare without a switch, as well as one light fixture disconnected when I arrived originally. The previous guy taped together a group of 5 or so white wires and 5 or so black wires, and one white wire was left desperate. It all looked weird and bunched up but I assumed white to white black to black ground to ground must be this white wire. Hook It up, hook up a light switch, flip the breaker back on and the light is on, but the new switch doesn't work. Not receiving the signal. Okay, call the homeowner, confirm the missing box was a switch for the light, she said no it was a flat plate. Flip the breaker off, disconnect the switch. Put it back on and go to flip the switch that she says is the one for the overhead I was installing, and the thing shorts the whole upstairs. I'm lost, I really hope all this detail helps because I have no idea and don't want to call an electrician just for a fixture
Every white wire is not a neutral. If you see an "extra" white, you should not connect it to a bundle of whites (which are neutrals) in the box. The extra white must have been a hot -- either switched hot or an always hot. If you know which white was originally not connected, then disconnect it from the other whites.
The previous guy probably also improperly connected a black to a group of other blacks. The proper connections might be to leave the white disconnected in the box with electrical tape over the end. Or the proper connection might be to connect this extra white to a certain one of the blacks. This might restore the circuit to functioning, but at this point it would be for the best to call an electrician.
You should not have added the white wire to the bundle of whites. Remove it. Even worse, "the last guy" that you're following isn't very good, and did a botch job - first he used an obsolete technique, and then he did that in a way that is statutorially wrong (breaks the rules but not a safety issue, except that it confused you and led you to do the wrong thing and blow the whole upstairs and possibly break other stuff, so yeah, safety issue. Code is Code for reasons).
New cable will need to be pulled from the switch to the lamp location, because it won't pass inspection the way it is. It will need to be /3 cable (black red white), and in that case, colors will match up after all. With red being the color of "switched-hot", which connects switch and lamp. You cannot run a single additional wire (don't even think about it), it has to be a full new /3+ground cable. The /2 cable can be reused if it can be gotten off cleanly.
Since the work was already shoddy, touching it was probably a strategic mistake, since now you're on the hook for it. Even after you skill up on electrical, I wouldn't touch that "electrician's" work anymore.
In the future, when connecting a lamp, lamp neutral generally goes to the bundle of neutrals and lamp hot generally goes to the "desperate" loose wire. However in a Code compliant situation, that wire will be black or red.