Linked files go missing and appear pixelated

I am new to illustrator and am having a hard time understanding how some of the elements in my files appear to go missing and will then appear low res and pixelated - I am working off a design template someone created for me and some files, seemingly random, display as blocky pixels. The strangest thing is that when I open the same file in preview on Mac it looks totally fine, even the original template I had sent to the print company in the past and had printed fine looks great on the pdf but when I open it in illustrator it turns into a blocky mess - I searched on the topic but many of the solutions I find appear to be on older versions of illustrator as when I try to navigate my menu and 'view' options I do not have the same options displayed as some of the turorials describe - any help on this would be highly appreacieated, thanks so much in advance!