If your access control system is using cards or fobs, make sure they are clean and free of debris. If the card reader is dirty, the system may not be able to read your card. Also, try waving the card in front of the reader instead of inserting it in. If you’re using a keypad, make sure you are entering the correct code and that your fingers are clean.
If there’s a problem with the system itself, then you’ll need to contact your vendor for support. If there’s a problem with user cards, then you might need to replace them or recalibrate them.