Issues with recharging 2008 Toyota Yaris

[ItsanEE]

My battery died sometime in the past three days (I use it casually). The weather here has been pretty cold but even last year I didn't have any serious problems starting. No light came on my dashboard last time I used it. Last night I tried to use my car and no lights would power on, my immobilizer light was off and my key would only work manually in the doors. My friend came to give me a boost and when we tried the headlights turned on and the horn blared continuously. Fuses appeared to be intact. The indicator on the battery for battery life should be either red(needs distilled water), blue (okay), or white (needs charging) however it is black. The level is at full so I'm wondering if it's either my battery or alternator. Either way, if I boost it again to take to my mechanic how do I get it to stop blaring the horn? I'm assuming this is related to the immobilizer and that there is some sequence to go through.

Any suggestions?