abs sensor fault

hi all first post here , just looking for a little help , i bought an a6 it's a 94 a6 the v6 one leather air con etc job , but had to replace the front nearside strut as the previous owner had rung 2 wheel studs , got the young lad at the garage to do it , BUT he forgot to re- fit the abs sensor and left it hanging over the hub ,i drove it a few hundred yards it wasn't damaged and i refitted it but now the car won't recognise the sensor or something along those lines as the the abs light and a large warning light comes on when the car is driven 25 metres or so , is this a computer reset job or is there something else
any help appreciated