How to undo accidental merge/migration of iPhoto to Photos via clicking OK to rebuild Photos?

Long story, but I have been using iPhoto all this time. I don't use it often. I RARELY used Photos except to print a single photo of say, an insurance card or other some such quick matter. Well, tonight I was going to print a set a pictures of objects that I'd been emailed on a contact sheet to make some flash cards, and I downloaded the photos in that email in Apple Mail to Photos (instead of downloading them to my hard drive, and then opening iPhoto and adding them to the iPhoto library).

Long story short, I clicked on Photos to begin my project, and got a window saying my Photos library needing rebuilding. Like a dummy, I hesitated for a second but then clicked OK (because I knew I only had around 20 meaningless photos in there), but now it seems many of my keepsake photos in iPhoto have "merged" into Photos! -- Except it's evident many of my pics have gone missing. Also, the photos I had downloaded to Photos from Mail are NOT there. More troubling, iPhoto now will NOT open, saying its contents have been merged into Photos. SO frustrating I was not given a warning that "rebuilding" Photos was also going to merge iPhoto into Photos, because then I NEVER would have clicked OK.

So, needless to say, I'm hoping to undo this action, and just put everything back to the way it was.

I do, thankfully, have a Time Machine backup that is two weeks old on an external hard drive. But, I'm not very good with computers anymore and have turned into my grandmother, never wanting to upgrade anything because I have no interest in learning new tricks. But it's painfully obvious now that this way of living has come home to bite me in the keister.

Is there a way to simply trash the Photos application and just go back to the last restored version of Photos from 2 weeks ago via Time Machine? Or is my best recourse to simply restore my entire laptop to the last saved version of everything from 2 weeks ago? Or, is there a way to undo the merge by using some fancy keystrokes in Terminal?

Ideally I'd like to keep everything current and not lose all my web history/bookmarks, downloads and two weeks of email, and just put back the iPhoto and Photos the way I had it before I clicked OK on rebuilding Photos... but I also know beggars can't be choosers.

Sorry I'm such a pathetic loser. Any help someone is willing to provide would be awesome.

Thank you Support Community!