I did that once and put it into the ATM, I freaked out, knowing I forgot to sign and went in to talk to the teller, she said they would just deposit it. They did, but if you are concerned you could go talk to them in the morning.
Usually, and if the bank it paying attention, they will not cash a check that you forgot to sign. I've taken checks from my boss before that he forgot to sign and even though they know him and me, they won't cash it.
Signing a check can mean two things. If the check is made out to you and you forgot to sign the back when you put it in the ATM, you're fine. If you wrote the check to someone else and forgot to sign the front, no bank will cash it.