I'd check all the breakers/fuses in the house. Stranger things have happened (especially in multiunit retrofits) where circuits are pilfered from another unit. Bob's suggestion of checking any GFCI is good as well.

Note that you really can't be legally working on the wiring in a rental situation. Call an electrician if playing with the breakers doesn't fix it.

May have to check the connections in the outlets and junction boxes, there may be a loose wire. I have lost service in some outlets in a branch circuit before and thats what it was. Some novice did not twist the wires before applying a wire nut. A backwire could possibly have came out also.

There's no requirement to pretwist the wires. If it came apart, it wasn't installed properly and pretwisting isn't necessarily going to help (it may end up just burning the wiring instead).

Again, if resetting breakers and GFCI doesn't fix the problem, you have no business screwing around with it any more than it already is on a rental property. If the outlets are fed from a breaker for another unit, that's illegal (in most cases) too, and needs to be fixed

I have respect for you and understand you know more about electric than me, but I completely disagree with that. Most of the junction boxes I found in my basement had untwisted wire before the nut applied. Some of the wire came out on their own without untwisting the nut, some of the nuts I could not untwist because arcing had welded the ends to the nut. Not twisting before a wire nut is just a bad idea, you want them nice and tight before the nut, then the nut just keeps them together. Only using the nut to try to make them tight is not a good idea.

You're completely wrong. Your untwisted wiring failed because it was installed improperly which wasn't because it wasn't pretwisted. You can make your anecdotal assertions all you want but neither the manufacturers of these connectors nor the listing agencies nor the electrical code says pretwisting does anything. All it does is give you some temporary illusion of a connection prior to getting the proper nut installation on.

If you want to twist fine, but it's not required and it doesn't guarantee (or even give a better chance) that things won't fail.

If wire ends are welding themselves to the nut, something else is definitely going on in there!

Back to the original topic... the OP said "my lower apt." so it's not clear whether the OP and the folks below are both renting, or the lower apt. is the basement of the OP's house that is being rented out as a separate apt. In which case the OP can do all the electrical work they want, I would think.

You think wrong. If the OP has tenants he has no business subjecting them to his amateur electrical work. I've never seen a local code that permitted this other than in single family, owner-occupied structures.

"If wire ends are welding themselves to the nut, something else is definitely going on in there!"

Yea, there was not a good connection because they were not pretwisted. The ones that I pretwist dont get loose, and never will. Just make sure you pretwist clockwise so the nut doesnt work against it. People that claim pretwisting might make it worse are twisting it the wrong way. FYI Ron you are not alone, there are plenty that dont believe in pretwisting and there are plenty that do.

Some good discussion here:

http://www.mikeholt.com/forum/Forum1/HTML/005609.html

"I
was taught to pretwist. I still pretwist with solid wire. I have seen
wire nuts melted because of poor connections. By pretwisting your not
depending on a wire nut as the sole mechanical connection of the splice.
Good connections means no call backs"

"Like
other on this board I have gone back to pre-twisting especially solid
wire. I find it works quite well and as mentioned fewer call backs
because something has come loose.
But after looking at the boxes of
wire nuts, listed in the instructions, all it says, " It is UN-Necessary
to pre-twist", it doesnt say it not recommended or it is not code
compliant just that it is UN-necessary, or more succinctly it is a time
saver not necessarily better not to."

Replacing one anecdotal statement with another doesn't make it true. You ddidn't bother to read all the followup statements where that statement was rebutted by the experts on Mike's forum?

There are people on both sides of the fence as I said. There is back and forth discussion but neither opinion is particularly wrong so we can agree to disagree. I quoted a few of the many pretwisters that I thought pertained to my opinion and the first quote I have seen happen first hand so it isnt just me. Anecdotal or not, there are plenty that agree with me and dont trust non-twisted connections.

Fellas, thank you all for your advice. All of your posts are a lesson. I have to call my electrician. None of the breakers tripped then wouldn't reset. Now, only one of the basement lights turn on. I'm afraid the tenants are attempting to do me 'a favor'. Or themselves. lol

My guy did have two 'apprentices' with him, so, I believe you all when all of you say nothing was done right in the beginning.

In case he doesn't cover this (I think he will - he's a standup guy who'll show up late at night if he has to), can any one here guesstimate a possible cost and say, for example, it's a lot, can anyone tell me how safe it is?

Thanks fellas!

Depends how long it takes, wont be able to guess. As far as safety, you need the correct tools and knowledge to do it. You can speed up the repair by finding all outlets, switches, and junction boxes and having them accessible. Hopefully, some hack didn't stick a junction (boxed or not) behind the wall somewhere.