993 is known to draw a small current even when not used, probably due to immobilisers or something. On top of that the battery is probably quite old, and therefore won't hold the charge as well. It should hold charge for at least 3 wks and start without problem, if not then the battery's probably had it.
Open the door and access the battery with key and reacharge it, and as soon as there's enough charge you should be able to use the remote to switch off the alarm. Alternatively you can jump-start it. At least that's how I dealt with mine, but may be different alarm has different "procedure".
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