Depends on year and engine of astra really, if it's a x17dtl [i think] then the alternator is on the back and has a vacuum pump attached iirc.

If it's a y17dti [identifiable by ecu right on the top of the engine] then alternator is on the front and comes out through the wheelarch i think. You can get them from motorfactors but i think it's a bosch only part soo if your factor only lists delco remy for example expect it to be wrong, priced at about £70.

I've not worked on one of these beasts for a while though so don't go hounding my if i've got the engine codes the wrong way round

The power steering has gone stiff as it's driven by an electric pump, if the battery has died due to not being charged then thats why.

Check alt is charging and the fuse hasn't blown as said above.

HTH