External harddrive not recognized

[S.t.a.c.e.y.]

Hi. I have recently upgarded my OS and since then cannot seem to get my MBP to recognize the WD ext. hard drive I was previously usisng. It does not shw up in Finder, or Disk Utilities. And ideas? I am away at moment, so dont ahve access to another comp to see if it is a h/drive fault, but seems unlikely... Thanks