Charged battery and now won’t start

Our 2000 Mustang is a weekend car at
most sometimes weeks go by with no use.
After a cold snap I tried to start but battery
was low. I removed the battery and
charged it with a slow trickle charge over
night. Good charge now turns over but will
not start and the theft light blinks when key
of off and out of the car. I have tried
numerous suggestions and even reset the
pcm. Checked fuses, grounds, have no
fault codes when I plugged in my obd2. I’m
really at a loss I can stop the blinking light
but still won’t start any suggestions would
be greatly appreciated. God Bless you all