Any idea why I can't see my objects in Solid mode? If I take a picture from the exact same position, they are visible in both material preview and rendered mode.

Hard to tell from the screenshot, but it almost looks like you accidentally used the ALT+B hotkey combo. This "clips" the view:

and when you change the viewpoint now:

Actually very handy in some situations! (This doesn't affect Rendered or Material Preview Mode...)

You can restore from that by again pressing ALT+B.

Thank you!!! Very useful to know also.

Hey alendomata ,

Check the eye icon in the properties panel for your objects/collection.

This is the hide in viewport option, hotkey H and ALT H to unhide. 

Note: Still visible in rendered and material preview mode.

And SHIFT H to hide the children of an object, but you don't really need to worry about this one.

Hope this helps.

Thank you @adrian2301 for the response. It was actually what spikeyxxx said, that I had accidentally pressed ALT+B.