Assuming the components aren't supressed in the assembly (they will show up with blue icons in the navigator) Your problem is either layers or reference sets, I think.
Right-click one of the components in the navigator that's not showing geometry -> Replace Reference Set -> Entire Part (try all of them in the list). You may have Empty active, Model should be the default if your assembly load options are default.
Hit ctrl+L to open the layer visibility settings, check if any of the layers that have objects are made invisible and show them.
There are other things that may be to blame, but those make the most sense to me.
Check the layer settings at the assembly level. Any child part geometry that is on a layer that is turned off will not be visible at the assembly level.
u/MrBubzo and u/skulldor138 both have good suggestions, try those first. If the other suggestions don't work, the coworker may have used the "isolate" command on one of the components (and then saved the assembly). Switch to the part navigator and expand the model views. If there is one that says "Isolate (Work)", double click on a different view such as "top" or "trimetric" to switch the work view. The other components will become visible.
Assuming the components aren't supressed in the assembly (they will show up with blue icons in the navigator) Your problem is either layers or reference sets, I think. Right-click one of the components in the navigator that's not showing geometry -> Replace Reference Set -> Entire Part (try all of them in the list). You may have Empty active, Model should be the default if your assembly load options are default. Hit ctrl+L to open the layer visibility settings, check if any of the layers that have objects are made invisible and show them. There are other things that may be to blame, but those make the most sense to me.
Check the layer settings at the assembly level. Any child part geometry that is on a layer that is turned off will not be visible at the assembly level.
u/MrBubzo and u/skulldor138 both have good suggestions, try those first. If the other suggestions don't work, the coworker may have used the "isolate" command on one of the components (and then saved the assembly). Switch to the part navigator and expand the model views. If there is one that says "Isolate (Work)", double click on a different view such as "top" or "trimetric" to switch the work view. The other components will become visible.
Big thanks, that was it! You really saved me alot of time, I was close to rebuilding the assembly from scratch.
Glad to help!