Relinking all my folders in LR

Ive just set up my Mac and I previously had a windows pc. Ive saved my photos, LR Cat and previews to a hard drive. When I open up the LR Cat all my Albums have the "!" logo. How to I resync my albums as its not giving me the option!!!!! Is it something to do with how I saved on PC to how I'm looking at in on a Mac. I have no  idea and I'm rubbish with computers