I bought lightroom 6 last year and everything has worked fine up until now...
Today I bought a new camera an 80d canon. I imported the images over to lightroom which told me that it could not be opened. A little research told me to update to at least the 6.5 lightroom version as I was running the 6.0 on my windows computer. The computer just would not update it so I uninstalled the whole program to reinstall it using the disc and serial number thinking it would download the most recent update. All worked fine but it was still the 6.0 version then I clicked on the update tab and this time it actually started updating so I thought I was actually getting somewhere! But no, I now cannot open lightroom at all it just tries to make me sign up to the free trial. Now it is trying to make me buy it again and I cannot see where to put my serial/keycode in to get it back. I've paid for lightroom 6 I just want to be able to use my new camera with it. Help!
Two things to try:
First download and install the update to 6.8 from here: Install Photoshop Lightroom
Your LR probably shows as CC2015 when you call the info from the Help menu.
If it does, you need to convert it to a standalone LR6: How to convert a Lightroom CC trial to Lightroom 6