What happens if you forgot to renew car registration?

your car will be unregistered
you will get a ticket if pulled over, but if you notice before you get this ticket just renew and nothing will happen.
The little Month sticker you put on your license plate expires and you could get a ticket for it -- I think that it's a fine + "fix it" ticket. If you want to pay it now (and it's late) I think that the DMV charges you a fine, in increasingly larger increments as the payment gets later. Check the DMV site in your area. Here's California's:
You could get a ticket if stopped for any reason by the police. Renew and pay the penalty costs if there are any.
Most states help you out by sending you a bill in the mail every year - how can you forget getting those bills? I don't know what state you're in - but most of them will send you a notification in the mail - that you failed to renew and that you only have so much time before they will take your license away or something like that. Some states will fine you money for not taking care of this on time. Most states require a sticker of some kind to be applied to your license plate so the cops will easily be able to pull you over based on having the wrong color sticker on your plate. You could end up with a ticket and having to go to court and all of that fun stuff. I'd go to the DMV ASAP tommorrow and take care of the situation. There may be exceptions such as being in the military - if your registration expires and you're like assigned to a ship or gone to the gulf - I'm sure they are more lenient in cases like that.
You will have to pay a late fee, but take care of this as quickly as possible. Don't panic because this is not unusual.
Depending on the state you live in, and how long it's been expired, you'll get pulled over and just get a ticket, or your car will be immediately towed

Just go to the DMV and get it renewed. You'll have to pay a small late fee.
In Texas we have to go to the court house and renew it after so many days, I had a penlty fee for waiting to long.