Unplug Your Computer for and Electrical Reset
My 1981 745I did the same thing. Unplug the computer and wait about a minute before plugging it back in. The computer in my car was located under the dash on the passengers side behind the speaker. It is a black box with a wiring harness going to it. These cars seem to have electrical hiccups from time to time and unplugging the computer worked for me. If that does not work I would start checking the electrical relays under the hood and dash.