Ok, we'll assume you've tried the old power button also like Ian
mentioned? And Someone below suggested a longer push of the power
button to wake up.

The next things to check, (1) how big is your "swap" space? It needs
to be at least as big as your physical RAM for suspend to work
properly, but probably 1.5 times that size to be sure. (2) you need
to check and make sure swap is mounted. Open a terminal and type
"swapon -s". The return from that command should list the partition
and size of swap if it is mounted properly.

The errors you describe after trying to push a brightness key may
suggest an additional error, but I'm not sure if that could be a swap
issue or something else.