Unable to Check-in online.

Hi,
Wonder if anyone can help me out with this one.
Tried to do the online check-in today for my Vegas flight tomorrow but it just came up with :-
"Unfortunately, we are unable to check you in online at the moment. Please check in at the airport as usual."
I am aware that seat reservation is not set in stone, but I've had this seat booked for many months now. Does it mean that someone has jumped in before me and taken my seat?! It was upper deck at the rear, somewhere I was looking forward to relaxing.
Many thanks in advance,
AC
Wonder if anyone can help me out with this one.
Tried to do the online check-in today for my Vegas flight tomorrow but it just came up with :-
"Unfortunately, we are unable to check you in online at the moment. Please check in at the airport as usual."
I am aware that seat reservation is not set in stone, but I've had this seat booked for many months now. Does it mean that someone has jumped in before me and taken my seat?! It was upper deck at the rear, somewhere I was looking forward to relaxing.
Many thanks in advance,
AC