Yosemite won't boot any more after attempting to enable FileVault and TRIM
Yosemite apparently uses a new security feature called kext signing. This will prevent the old method of TRIM enabling. See the Trim Enabler FAQ, The question Recovering from stop sign on boot screen might help you:


  Recovering from stop sign on boot screen
  
  For those who are stuck on the grey boot screen, here’s how you get
  back into OS X: 
  
  Step 1: Boot recovery mode by holding Cmd+R during boot
  
  Step 2: Is your volume encrypted (FileVault)? If so, first open Disk Utility from the menu bar and unlock/mount your drive. [see here for the procedure]
  
  Step 3: Open the Terminal from the menu bar
  
  Step 4: Run this command: nvram boot-args
  
  Does it say kext-dev-mode=1? if so, you can skip to Step 6. If it
  says error getting variable, continue with these steps:
  
  Step 5: Run this command: nvram boot-args=kext-dev-mode=1
  
  Step 6: Reboot back in to Recovery Mode again
  
  Step 7: Run these commands, replacing Your Disk Name with the name of your Mac disk
  (partition). You can type ls /Volumes to get a list of volumes. Note
  the quotes around the disk path and that there should be no / before
  System.
  
  cd "/Volumes/Your Disk Name"
  
  touch System/Library/Extensions
  
  kextcache -u "/Volumes/Your Disk name"
  
  Step 8: Wait until it finishes (can take as long as 5-10 minutes,
  don’t abort it) and reboot. You should now be back in OS X with Trim enabled.
  
  The commands are not working!
  
  The commands will work if they are entered correctly. However, if you are not familiar with the Terminal, it can be hard to follow the steps above. If can’t get the Terminal commands to work, or want an easier option, you can also use the Recovery Mode to reinstall OS X. Reinstalling OS X will only reinstall the system files, leaving your files and preferences untouched.


Please check the original source for updates!

EDIT:
See my answer here: Since the release of 10.10.4, Apple now provides a new tool called trimforce, allowing users to activate TRIM also on unsupported disks. No more dangerous experiments with third party software necessary.
/EDIT