Help! How to deactivate car alarm?

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I disconnected the battery for a couple of weeks to do some repair work
on my truck. When I reconnected battery after the repair was done, I
found an alarm system was activated. It's preventing me from starting
the engine. I didn't even know the truck was equipped with an alarm
system before the incidence. The previous owner probably deactivated it
when he sold the truck to me. I don't have the remote and alarm
manual. I am wondering if there is a way to deactivate the alarm again.
The alarm is an after-market device. I have no idea about the wiring on
it. If you know how to fix this issue or where I can find help, please
let me know. Thank you!