Ubuntu gone after updating Mac OS X After installing updates for Mac, rEFInd won't show up when the computer starts, and even if I held down the Option key the disk manager only shows Macintosh HD. I already tried disabling csrutil and also reinstalling rEFInd, none of those work. 

When I held down Option-Command-P-R then opened disk utility, it showed both Macintosh HD and disk0s3 (name of my ubuntu partition) but disk0s3 was greyed out. 

How do I get into ubuntu again?

Version: Mac OS High Sierra

Ubuntu Version: 16.04 LTS
I solved it! My mistake was trying to install another, newer version of rEFInd at first without completely removing my previous rEFInd folder. I then deleted my newly downloaded rEFInd folder then simply installed my old rEFInd version again.