Possible reasons:
- remote’s battery is low
- remote is defective
- vehicle’s battery is low

If the car alarm doesn’t trigger the alarm after open the door and/or LED isn’t blinking then there are two variants:
alarm power disconnected, at this case recommend you to contact to installers for diagnostics.
vehicle’s battery is low, requires to charge or to replace battery

If alarm is activated and/or LED is blinking then at the first replace the remote’s battery to new.
If battery’s replacement doesn't help open the door by key. If car alarm triggers the alarm then disarm by service button according to the instruction.
Service button is usually in typical places:

Service button: