iMac wont wake from sleep

i have the most recent 21.5" iMac (late 2011) and it almost never seems to want wake from sleep if i havent used it in a day or so... it forces me to force it to shut down by holding power the button down. every now and then i get an error that makes my mac crash and forces me to restart it... next time i get this error i will post it here... any help would be greatly appreciated.