Be sure to disconnect any third-party external devices. Try booting into Safe Mode.
You can do this:
Repair the Hard Drive and Permissions - Yosemite, Mavericks, Mountain Lion or Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.
Repair
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD entry from the the left side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu. Select Restart from the Apple menu.
You may have some success with this:
Try these in order testing your system after each to see if it's back to normal:
1. a. Resetting your Mac's PRAM and NVRAM
b. Intel-based Macs: Resetting the System Management Controller (SMC)
2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the
COMMAND and R keys until the Utilities menu screen appears.
3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the
indented Macintosh HDentry from the the left side list. Click on the First Aid button in the toolbar. Wait until the operation
completes, then quit Disk Utility and returnto the main menu.
4. Reinstall OS X: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.