Most of my apps won't open including app store. App store reads, App store quit unexpectedly...then never opens. So frustrated here! Anyone to help?

Hi! I think you need to upgrade to OS X 10.10.5. I'm almost sure this will solve all the issues.

Try the combo update:

OS X Yosemite 10.10.5 Combo Update

I see. Try the steps below and then try updating using Mac App Store.

Restart your Mac.

Try Safe Boot:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- After your Mac boots up, restart it as you usually do.

If this doesn't help, follow the instructions below:

- Shut down your Mac

- Wait until your computer turns off and after that press the Power button

- Right after you hear the startup tone, press and hold the Shift key

- Release the Shift key when you see a grey Apple sign and the progress bar below this sign

- Once you see Desktop, start a Disk Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)

- Click on Verify Disk and then, if asked to fix problems, on Repair Disk

- After this, click on Verify Disk Permissions and then on Repair Disk Permissions

- After the process is finished, shut down your Mac and turn it back on after about 30 seconds

Apart from that, take a look at this Apple article and follow the instructions on Resetting NVRAM shown there:How to Reset NVRAM on your Mac - Apple Support

Pleased to hear this helped you! You are welcome.