In OS X 10.10 (Yosemite), Apple introduced a new security requirement called kext signing. Kext signing works by using Apple-granted signatures to verify system integrity. If a driver has been modified, Yosemite will no longer load it. Trim Enabler 3.3 and older versions worked by modifying the system driver responsible for disk management. This method of enabling Trim could render users systems unable to boot.
Enabling Trim safely
After this issue occurred we sat down and developed our own custom driver to enable Trim in Mac computers. After discussions with Apple representatives, we were also granted an official kext certificate to ensure that our driver would load safely on Yosemite and all future versions of macOS. You can now enable Trim safely using Trim Enabler 3.4 or Disk Sensei. These apps will never cause any "grey screen" issues.
Issues with using older Trim Enabler
If you try to use an older version of Trim Enabler (3.3 or earlier), you could end up with the “grey stop sign” screen when trying to boot. To avoid this situation, use Trim Enabler 3.4 or later, or the new Disk Sensei app.
I accidentally used an older version of Trim Enabler and now I can't boot
If you are stuck with a gray boot sign, the easiest way to recover is to reinstall macOS:
Note that reinstalling macOS (without formatting) will not delete any of your personal files.