It's hard to tell in the picture, but if you take the cap off what does the part left behind look like? My guess is you either have an allen head set bolt, of a shear pin that broke.
Does the nub left in the shaft go clear through the shaft? If it goes clear through you should be able to use a hammer and punch and gently tap the pin out of the shaft. Then go to Home Depot or Lowes and get a new pin to hold the cap on.
If it's a set bolt you might be able to back it out and either get a new longer one or even a regular bolt to hold the cap on.
Hopefully it's not a spot weld or you'll probably have to get someone to repair it for you.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup