iMovie wont open

You need to upgrade your operating system from Yosemite to El Capitan or Sierra, and upgrade your iMovie version to iMovie 10.1.2 that is the most current version.

As a side note, some posters on this forum have reported problems running iMovie after upgrading to Sierra. Just wanted you to be aware of that.

You can check with the Apple app store to see if you can update Yosemite for free. You may be able to update it to Sierra for free, but you probably will not be able to update to El Capitan for free because Apple no longer sells El Capitan, so you would need to buy it from a third party vendor like Amazon or ebay. I checked Amazon and I believe they have El Capitan for $15.00.

I would first update your iMovie version to current version 10.1.2 and see if that runs on Yosemite. If it still doesn't, then you can upgrade to El Capitan or Sierra in whatever way is available.