reset the alarm:
disconnect battery. reconnect after 5-10mins.
(make sure the battery on the key fob is in good order) shut down everything. close all doors. lock the care with the key.
then open with the alarm keyless entry... and you should be fine.
** if it keeps doing that and you are using an aftermarket alarm on a bimmer, i suggest u uninstall it. the keyless entry/alarm will only cause you lotsa trouble down the road.