Garry's mod won't load

Hi everyone and thanks in advance for any advice here. I have a homebuilt computer running win7 3GB. My nephew been playing Gmod on it for awhile. My Geforce 512 MB video card burned out. It was replace by a 1 GB AMD I believe. He played Gmod for a day and now it wont load. Other games play ok. AT first it just said loading… forever. I haven’t seen this but nephew said something about video card not in data base? Not sure if that is an error he is getting or someone online told him this. He did play for awhile with new card. Tried reinstalling. Other games run fine. Any ideas?