I believe you have a keyless entry/remote on it, the system got reset and is beeping.
1. Open the driver's door, and insert the key into the ignition. Leave the door open.
2. Remove the key from the ignition, and within 5 seconds, re-insert the key and then turn the ignition to ON.
3. Wait at least 10 seconds, but less than 20 seconds. After waiting, press a button on the remote twice. Press the same button two times - don't press two different buttons. The system will acknowledge with a beep if you did it right.
Toyota's instructions for this sort of thing are very literal and must be followed EXACTLY. It's helpful to use the flashing ":" symbol in the clock to count the seconds in step 3.
If you disconnect the battery, the setting will return to the factory default setting of ON.