Typically, your alarm system will be set up with doors and windows labeled as certain zones. Zone 1 might be the front door; zone 2 might be the back door; zone 3 might be your bedroom window. Your alarm manual should describe what each zone is. Typically, the system will display an error message such as "sensor open" or it will simply display the zone number. Check and make sure each zone is closed. This means you need to make sure that all the doors and windows are secure. Even if a zone is partially ajar, the alarm system will not arm or work properly.
Most alarm systems come equipped with a battery designed to supply power to the system in case the main power to the house goes out. If this battery gets low, the alarm system may start to beep. If you notice the alarm system is beeping, ask yourself if you recently experienced a power outage and check and see if the system is displaying "system trouble" or "low battery" signs. You may simply need to input your code and arm and disarm the system to clear it. When the power is restored, the battery should recharge and the beeping should stop. If it does not, call your alarm company; the battery may need to be replaced.
If you believe that your code is not working, try inputting the code again. Most often, the reason is a simple mistake in inputting the code. Make sure that you press hard on the keys; it could be that the key pad is getting old. If you are still experiencing trouble, contact your alarm company; the mistake could be on its end. Explain the situation and see if someone there accidentally did anything to the system. If not, a technician can come out and check your system.