Marketplace button often is not there when I start the program.  Only seems to come up when there is a special offer or free item.  During the last special offer, I actually went in and bought a texture pack.  Works fine, but saw others that I want to try.  I cannot get back to the Marketplace until the button shows up again.

Not sure how long it has been doing this as I did not care before I wanted to buy something.  But I've noticed it for a few weeks at least.

I have not tried to reboot.