One possible issue is that there may be old leftover simulators installed, which are not compatible with the new XCode, and their presence causes the whole Simulator to fail. To get rid of them, delete the simulators in /Library/Developer/CoreSimulator/Profiles/Runtimes (for me there was an iOS 7 simulator there which was the issue). Relaunch XCode after this.
Then try to run the simulator from XCode->Open Developer Tool->Simulator. Assuming this succeeds, verify that you have the simulators under the iOS Simulator menu Hardware->Device->Manage Devices… – if not, you should be able to click the + in the bottom bar to add yourself some iOS 9 simulators. Relaunch XCode again and they should show up in the menu.
In my case was with Xcode 8.2, what it worked to me was changing Deployment Version:
After setting another different than 10.0 then appeared back all simulators again.
Have you tried going to Xcode > preferences > downloads and trying to re-download the simulators you want?
For me it was only showing one simulator,
and when I go to Xcode > Preferences I can see the simulator is already downloaded but it is not showing up in the list,
if you're facing something similar. This is how you should add required simulator,
From the Xcode menu, open Windows > Devices, shown in the image below
you should see this screen,
notice there is only one simulator (ref: first image)
Now to add required simulator, you should click on the + (in the left corner)
You should see this,
You can choose the simulator from the list, In my case it is iPhone 6s Plus.
In Xcode you can see additional simulator iPhone 6s Plus,
I was also not able to see simulators.Reason is,I have not restarted system after upgrading Xcode.I restarted & simulator started displaying as routine.
There is a bug in the latest El Capitan beta which causes a process to crash when dlopen() fails to mmap() a dynamic library. Such mmap() failures can occur due to code signature verification failures.
These mmap() failures will occur if older iOS Simulator runtimes are present on disk (ie: installed from Xcode 6.x), and that is the reason why Xcode 7 beta release notes have indicated that earlier runtimes are not working in Xcode 7. The iOS 8.x Simulator runtimes were updated in newer downloads, but if the older versions are present on disk, they will cause the mmap() failure, which in the latest El Capitan beta, unfortunately results in the CoreSimulatorService repeatedly crashing which in turn results in no simulators being available.
If simulators are suddenly disappeared:
Just consider decreasing Deployment target to a minor version below current version in Project > Build Settings > Deployment Info > Deployment Target
P.S. Usually you see a place holder text showing the selected version (in gray) in the box denoted above. If for example the gray text says 9.3, then decrease it to 9.2.
Look at seancook's response in the middle of this page... it seems to have fixed this issue for several people.
https://forums.developer.apple.com/thread/11706
Edit: This can also be fixed by booting into recovery mode (CMD - R) and reinstalling El Capitan.
Go to Windows. Select Devices. Select '+' from left bottom and add all simulators one by one.
I had changed the name of my Project from Finder. I clicked on the old project name (between the Play Button and and the selector for the desired Simulator or Device), selected edit schemes, selected the Targets Tab, and realised that the old scheme was missing so I just added my new Project name by clicking on the plus button and the simulator list reappeared.
