check the fuses then check high/low beam switch
It may be simply that you were running with only one low beam instead of 2 and both are now burned out. If not, then the high beam switch or relay went bad.
"It's possible that you blew the low beam filaments. I blew my high beam filaments while charging the battery." --MM
There should be separate fuses/relays for the low beams. Check the underhood fuse block located on the firewall. They are all clearly labeled.
Mounted to the top left side of the steering collumn under the intrument panel.
Type "1991 Buick photos" into your search engine. If you are interested in a particular Buick Model then add that model such as "1991 Buick LeSabre photo".
Pull the turn signal stalk.
Its the multiuse switch on the high beam/turn signal indicator. You can get a new switch installed for about $400.
Depends on what type of 1991 Buick you're referring to...
How to change brake lines on a 1991 Buick Lesabre?
Depends on what model Buick...
Try the floor to the front left. There should be a switch you can press with your foot.
Depending on the year, the Regal could have a relay to control the high beams. Check the distribution box under the hood to see if there is a bad headlight relay.