yes put ur old one back in , make sure engine works ok first.
then put new head unit in , if no start , then maybe head unit it faulty. or shorted fuse box so open ur bonet, or under neath ur steerin wheel or glove box will be a fuse box .... take each fuse out individually one by one and put against the sun light to see which one is shorted and reaplce
u will need to refer to ur car owners manual in order to find the location of the fuse box ... other astra owners on here hopefully will be able to tell u if u have lost ur car manual .