Power supply re-install - Now it won't boot in OSX

Hi

I removed and re-installed my PSU because I needed to test a different mobo and now I can't boot in OSX. I didn't change anything about the system - literally just took out the PSU and then put it back. It doesn't even give me the clover screen - just goes straight into Windows.

It shows the OSX boot partition, but when I select it just goes straight into Win10.
BIOS screenshot

Please help!

i7-3770K
GIGABYTE GA-Z77MX-D3H
EVGA 970 FTW Samsung SATA SSD

Also, If I need to reinstall, I do I need to reformat the entire HD or can I keep the Win10 partition intact?

I built this over 5 years ago and haven't needed to mess with it since, so I'd be pretty much starting over from square 1.

Thanks!