Unlucky timinig!
If you are not on PC, this is what I would do:
- Study this: http://www.uesp.net/wiki/Oblivion:Mages_Guild_Suspension
- Go get 20 of one of the ingredients you need (assuming this is your first guild suspension). The link can guide you to where you can quickly find them.
- Get reinstated in the guild.
- Return Erthor to the head of the Skingrad Guild.
If you are on PC, you can do the same thing or resolve it in several other ways using the command console.
Good luck!