For some reason the assembly has lost (or thinks it has lost) the reference to the part. You might try opening the part first, then opening the assembly, and see if it opens OK. If that works, then Save and close both files. Open the assembly and it should be OK. If that doesn't work, I would suggest re-naming the part, then open the assembly, right-click on the suppressed part in the tree, and choose "Replace Component". Replace the suppressed component with the re-named one. If that works, and you need to change the name back to the original, do that with SW Explorer so it doesn't lose the link again.
I had the same problem, All of my fastener hardware became suppressed in the various subassms and would not unsupress.
I opened each type of nut and bolt and then I was able to unsupress them.